Endotext.com - Endocrinology Of Male Reproduction, Chapter 5

SYNOPSIS

The clinical presentation, differential diagnosis and treatment of hypogonadotropic hypogonadism (HH) in the human male are discussed. Particular emphasis is placed on the pathophysiology and genetics of HH, as well as the different modes of therapy to induce spermatogenesis.

DEFINITION

Pulsatile secretion of gonadotropin-releasing hormone (GnRH) from the hypothalamus is required for both the initiation and maintenance of the reproductive axis in the human. Pulsatile GnRH stimulates the biosynthesis of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) that in turn initiate both intra-gonadal testosterone production and spermatogenesis as well as systemic testosterone secretion and virilization. Failure of this episodic GnRH secretion or action or disruption of gonadotropin secretion result in the clinical syndrome of hypogonadotropic hypogonadism (HH). Disorders causing HH are differentiated from primary testicular disease by the demonstration of low/normal gonadotropin levels in the setting of low testosterone concentrations and sperm counts (Fig. 1). Congenital abnormalities leading to HH are rare but well described and are usually the consequence of deficient GnRH secretion occurring either in isolation (idiopathic hypogonadotropic hypogonadism (IHH)), or in association with anosmia (Kallmann syndrome (KS)). However, mutations in the GnRH receptor and in both LH-b and FSH-b subunits have also been reported. Acquired causes of HH are more common and can be due to any disorder that affects the hypothalamic-pituitary axis.

NORMAL GnRH SECRETION ACROSS DEVELOPMENT IN THE MALE

In the human, the pattern of GnRH-induced gonadotropin secretion is constantly changing across sexual development. Therefore, the establishment of a robust normative database is critical for understanding pathologic states like HH.

Fetal/Neonatal Life

GnRH neurons originate outside the central nervous system in the olfactory placode, migrate along the olfactory, terminalis, and vomeronasal nerves up the nasal septum, and through the cribriform plate to the forebrain, ultimately reaching their final destination in the arcuate nucleus of the hypothalamus (1). While GnRH neurons have been demonstrated in the fetal hypothalamus by 9 weeks gestation, it is not until 16 weeks that functional connections are established between these neurons and the portal system. From mid-gestation until 6 months of postnatal life, pulsatile secretion of GnRH stimulates gonadotropin biosynthesis and secretion that, in turn, initiates gonadal sex steroid production (2).

Childhood Period

During childhood, the hypothalamic-pituitary axis is not completely quiescent and is characterized by low amplitude GnRH secretion as mirrored by LH secretion using ultrasensitive LH assays (3).

Puberty

The onset of puberty is marked by sleep-entrained reactivation of the reproductive axis characterized by a striking increase in the amplitude of LH pulses with a lesser change in frequency (4). This nocturnal rise of LH secretion stimulates gonadal secretion of sex steroids, which return to prepubertal levels during the daytime. Concommitently, nocturnal FSH secretion also occurs and induces inhibin B secretion . As puberty progresses, secretion of gonadotropins occurs during both day and night, allowing sexual development to be completed. The precise neuroendocrine trigger to puberty is still unknown. However, it is likely to be a process that removes inhibition of GnRH release rather than one increasing GnRH synthesis as abundant GnRH mRNA is present in the hypothalamic neurons of primates at an equivalent developmental stage (5, 6).

Adulthood

During adulthood, gonadotropins are secreted in a pulsatile fashion. In the adult male, LH is secreted in pulses approximately every 2 hours (Fig. 2A) (7). However, considerable variability is observed in LH pulse patterns and there is a wide range of testosterone secretory patterns. Indeed, in 15% of normal men whose hypothalamic-pituitary-gonadal (HPG) axis was examined using frequent blood sampling, serum testosterone levels as low as 3.5 nmol/L were recorded (to convert to ng/dL, multiply by 28.6) following long inter-pulse intervals of LH secretion, although mean testosterone levels were within the normal range (Fig. 3, below). This within-patient variation must be considered when interpreting single LH and testosterone measurements obtained during the evaluation of a male with suspected hypogonadism.

Figure 2. Spectrum of GnRH-induced LH secretion in men with GnRH deficiency. LH pulsations are indicated by asterisks. A, Normal adult male pattern of GnRH secretion with high amplitude regular LH pulsations and normal serum testosterone (T), testicular volume (TV) and sperm count; B, Disordered amplitude pattern of GnRH secretion in an IHH male, characterized by low amplitude LH pulsations, low serum T, and azoospermia; C, Sleep-entrained or developmental arrest pattern of LH secretion in an IHH male characterized by relatively low amplitude LH pulsations clustered during the night-time hours analogous to the pattern which normally occurs at puberty. Note that the TV is higher than in the subject with the apulsatile pattern; D, Apulsatile pattern of GnRH secretion in an IHH male with complete absence of endogenous LH pulsations, low serum T, prepubertal TV and azoospermia.

Figure 3. Frequent blood sampling (q 10 min for 24 h) of serum testosterone (T) and LH in a normal male. Note that a T level of 91ng/dl (to convert to nmol/L, multiply by 0.03467) which is in the hypogonadal range was recorded after a long interpulse interval of LH secretion.

The clinical features of GnRH and/or gonadotropin deficiency are typically first manifested at puberty, a time when there is normally a marked increase in GnRH secretion. The phenotypic presentation of HH varies with age of onset (congenital vs. acquired), severity (complete vs. partial), and duration (functional vs. permanent).

HH may also be diagnosed in the neonatal period. The typical clinical phenotype is that of a male infant with normal sexual differentiation, cryptorchidism and micropenis in whom gonadotropin and sex steroid levels are inappropriately low given the normal activation of the HPG axis during this period. Early in fetal life, the testosterone production required for full sexual differentiation is thought to be stimulated by maternal hCG alone. However, endogenous secretion of GnRH in the late fetal/early neonatal periods appears necessary for inguino-scrotal descent of the testes and full growth of the external genitalia (8, 9). Accordingly, cryptorchidism and microphallus have been reported in up to 50% of patients with IHH and KS in some small series (10, 11) and may represent surrogate markers of failure of activation of GnRH secretion during the neonatal window.

Most often, the diagnosis of HH is delayed until adolescence, when there is a failure to go through puberty. The clinical presentation includes lack of development of secondary sex characteristics, eunuchoidal body proportions (upper/lower body ratio <1 with an arm span 6 cm > standing height), a high-pitched voice, slight anemia, delayed bone age, and pre-pubertal testes (11). However, the syndrome is clinically heterogeneous in that some patients have evidence of partial spontaneous pubertal development reflected by larger gonadal size despite hypogonadal testosterone levels and inappropriately low gonadotropin levels. Gynecomastia is not a typical feature of GnRH deficiency given the hypogonadotropic state and is most commonly seen in patients treated with gonadotropins (12, 13). HH may also present after completion of puberty resulting in a disruption in reproductive function in adulthood characterized by decreased libido, impotence and oligo- or azoospermia (14).

DIFFERENTIAL DIAGNOSIS

Hypogonadotropic hypogonadism may be broadly classified into congenital or acquired disorders (Table 1).

IHH is characterized by an isolated defect in GnRH secretion as evidenced by: i) complete or partial absence of GnRH-induced LH pulsations (Fig. 1, see above) (15, 16); ii) normalization of pituitary-gonadal axis function in response to physiological regimens of exogenous GnRH replacement (15-17); iii) otherwise normal hormonal testing of the anterior pituitary including a normal ferritin level and; iv) normal imaging of the hypothalamic-pituitary region.

IHH was first reported in association with anosmia and termed Kallmann syndrome (18). However, it was subsequently appreciated that several patients with IHH lack evidence of an olfactory defect and thus have a normosmic form of IHH. While the majority of IHH patients present with lack of pubertal development, there is considerable clinical heterogeneity. Depending on the degree of prior spontaneous pubertal development, testicular size in men with IHH may range from prepubertal to near-normal adult testes. In addition to anosmia, a variety of other anomalies have been reported to occur in IHH with an increased frequency in the KS subset including cleft lip and palate, synkinesia, sensorineural deafness, cerebellar ataxia and renal agenesis (18-20).

Given that it is a rare disease, data on the incidence of IHH is limited. The estimated incidence varies from 1/10,000 to 1/86,000 (21, 22). Isolated GnRH deficiency occurs more commonly in men than in women. Based on our review of 250 consecutive cases seen at the Massachusetts General Hospital, the male:female ratio is 4:1.

i) Adult onset IHH
Our group described an acquired form of isolated GnRH deficiency termed adult-onset idiopathic hypogonadotropic hypogonadism (14). In this group of patients, puberty occurs normally and is followed years later by a decrease in libido, sexual function and fertility. The biochemical profile of these patients is indistinguishable from subjects with congenital GnRH deficiency in that they have an apulsatile pattern of LH secretion associated with low serum testosterone levels. In addition, more than 90% of cases have normal restoration of the pituitary-gonadal axis when treated with physiologic GnRH replacement regimens supporting a hypothalamic defect as the origin of the disorder. Unlike patients with functional GnRH deficiency, no factors known to impair GnRH secretion transiently such as stress, exercise or weight loss could be identified in this population. In addition, longitudinal observation of these cases of adult-onset IHH suggests a permanent neuroendocrine defect.

ii) Fertile Eunuch Syndrome
In 1950, McCullagh et al provided the first description of a patient with the fertile eunuch syndrome characterized by eunuchoidal proportions and lack of secondary sexual characteristics in the presence of normal size testes and preserved spermatogenesis (23). In this disorder, enfeebled endogenous GnRH secretion appears sufficient to achieve the intra-gonadal testosterone levels needed to support spermatogenesis and testicular growth, but is insufficient to induce virilization. The clinical picture of the fertile eunuch is rather similar to that of mid-pubertal boys; indeed, frequent blood sampling in two men with the fertile eunuch syndrome demonstrated a nocturnal rise of LH and testosterone secretion synchronous with sleep, analogous to the pattern seen in mid-puberty (24). In contrast, we recently described a patient with the fertile eunuch syndrome displaying a detectable but apulsatile pattern of LH secretion, who was found to harbor a partially inactivating mutation of the GnRH receptor (GnRH-R) (25). The clinical presentation of the fertile eunuch syndrome also reveals some similarity to adult onset IHH in that both are characterized by GnRH deficiency in association with normal or near-normal testicular size. However, "fertile eunuchs" are distinguished by the preservation of spermatogenesis and the achievement of fertility with testosterone or hCG therapy alone (26, 27).

iii) Delayed puberty
Frequently, there is a history of delayed but otherwise normal puberty among the families of patients with IHH (20). In the general population, the incidence of delayed puberty is less than 1% (28). However, in our series of 106 patients with IHH, 12% had relatives with a history of delayed puberty (20). These data suggest that delay in initiation of puberty but subsequent normal progression through puberty may represent the mildest end of the phenotypic spectrum of IHH.

2. ACQUIRED

Functional

Functional forms of HH are characterized by a transient defect in GnRH secretion. This type of presentation occurs most commonly in female hypogonadotropic subjects with hypothalamic amenorrhea (HA). In susceptible individuals, HA may be precipitated by factors such as significant weight loss, exercise or stress (29-31). Typically, GnRH secretion will recur after correcting the underlying abnormality and menstruation will be restored. While in women the presence or absence of menses acts as a useful clinical marker of the functioning of the HPG axis, there is no comparable clinical marker in the male. Moderate to severe dietary restriction in otherwise healthy men has been shown to decrease testosterone levels by impairing secretion of GnRH (32, 33). In addition, some (34, 35) but not all (36, 37) studies have shown that strenuous physical exercise may adversely affect testosterone concentrations. However, a clinical syndrome of functional GnRH deficiency in men analogous to HA has not been definitively confirmed.

Drug-Induced GnRH Deficiency

In athletes, use of anabolic steroids may result in a functional form of HH. This is more commonly seen in men and is manifested by decreased concentrations of both testosterone and dihydrotestosterone associated with a marked impairment of spermatogenesis (38, 39). While the suppression of the HPG axis induced by anabolic steroids is reversible, it may persist for up to 16 weeks following withdrawal of the steroids (38). Chronic treatment with glucocorticoids may also lead to hypogonadism. In one study, 16 men with chronic pulmonary disease who received high-dose glucocorticoids for at least one month had a mean serum testosterone of 6.9 nmol/L, compared to 15.6 nmol/L in 11 men matched for age and disease (40). Given that serum LH levels did not increase in this study, these data suggest a predominantly central mechanism for glucocorticoid-induced hypogonadism. Chronic use of narcotic analgesics may also suppress LH secretion and result in reversible HH (41). A common side-effect of psychotropic medications such as phenothiazines is hyperprolactinemia which inhibits endogenous GnRH release resulting in HH.

Critical Illness

Any period of severe chronic (42), or acute illness such as surgery (43), myocardial infarct (44), and burn injury (45) may result in low testosterone levels (46). Acute injury is accompanied by a prompt and direct suppression of Leydig cell function (47). When severe stress becomes prolonged, hypogonadotropism ensues largely due to attenuation of pulsatile LH release (47). Endogenous dopamine or opiates may be involved in the pathogenesis of HH induced by critical illness (46).

Structural lesions of the hypothalamus and pituitary can interfere with the normal pattern of GnRH and/or gonadotropin secretion. The majority of patients with HH secondary to such tumors have multiple pituitary hormone deficiencies in addition to that of gonadotropins (48, 49). However, a mass lesion in the pituitary or hypothalamus is more likely to disrupt the secretion of gonadotropins than that of ACTH or TSH. Thus patients may present with hypogonadism in the absence of adrenal or thyroid hormone deficiency.

In children, craniopharyngioma is the most common tumor resulting in HH, and is often associated with growth retardation, visual field defects and diabetes insipidus. In adults, prolactinomas are the most frequent cause of HH and may do so by either interfering with GnRH secretion, or in the case of macroadenomas, by local destruction and compression of the gonadotropes. Hyperprolactinemia results in altered dopaminergic function, which has been shown to reduce GnRH mRNA levels and decrease serum levels of LH, FSH and testosterone (50, 51). Although men with hyperprolactinemia may develop galactorrhea, it occurs less frequently than in women, presumably due to lack of prior stimulation by estrogen and progesterone.

Rarer causes of HH include infiltrative disorders of the hypothalamus or pituitary such as hemochromatosis (52), sarcoidosis, lymphocytic hypophysitis (53) and histiocytosis, in which case the presence of systemic signs and symptoms frequently leads one to the diagnosis. Cranial irradiation for the treatment of CNS tumors or leukemia may also result in the gradual onset of hypothalamic-pituitary failure (54-56). The degree of impairment depends upon both the dose and type of radiation employed and typically reflects hypothalamic dysfunction since the hypothalamus is significantly more radiosensitive than the pituitary. In general, the younger the patient the greater the susceptibility to endocrine dysfunction following radiation therapy (57). Sudden and severe hemorrhage into the pituitary can also results in permanent impairment of pituitary function including hypogonadism (58).

PATHOPHYSIOLOGY

The combined use of both frequent blood sampling and genetic studies have contributed to our understanding of the pathophysiology of isolated GnRH deficiency in the human. Due to its confinement within the hypophyseal-portal blood supply, direct sampling of GnRH is not feasible in the human. In addition, measurements of GnRH in the peripheral circulation do not accurately reflect its secretion due to its rapid half-life of 2-4 min (59, 60). Consequently, inferential approaches must be used to study GnRH secretion in the human.
Traditionally, LH has been used as a surrogate marker of GnRH activity (61, 62). More recently, the pulsatile component of free alpha subunit (FAS) secretion has been shown to be tightly correlated with that of LH (63) and to be driven by GnRH based on its eradication by GnRH receptor blockade (64). Given its half-life of 12-15 min, FAS is useful in tracking GnRH secretion at fast pulse frequencies (65).

Patterns of GnRH Secretion in Subjects with GnRH Deficiency

A range of abnormalities in the neuroendocrine pattern of GnRH secretion mirrors the clinical spectrum of IHH. We examined pulsatile gonadotropin secretion in 50 men with isolated GnRH deficiency during 10-min blood sampling for up to 24 h (15, 16). The largest subset of patients (84%) had no detectable LH pulses (apulsatile pattern, Fig. 2D) and were found to have neither historical nor physical evidence of puberty, thus representing the most severe form of GnRH deficiency. A second group of subjects demonstrated predominantly nocturnal LH pulsations (developmental arrest pattern, Fig. 2C); these patients had some testicular growth and a history consistent with an arrest of puberty. Another pattern of GnRH secretion was observed in which LH pulses occurred at a normal frequency but were of diminished amplitude compared to those of normal men (decreased amplitude pattern, Fig. 2B). This decreased pulse amplitude pattern is suggestive of either enfeebled hypothalamic release of GnRH or a state of GnRH resistance as may be seen with a partial defect at the level of the GnRH-R. Finally, pulsatile LH activity appeared normal by RIA in one patient; however, LH bioactivity was absent when tested in the dispersed rat Leydig cell assay (66). Thus in IHH men, the spectrum of abnormalities in GnRH secretion or action is likely to contribute to the clinical and biochemical heterogeneity of this disorder.

Genetics of IHH

Considerable genetic heterogeneity has also been found to underlie IHH. Most cases of IHH are sporadic (80%), suggesting that either the frequency of spontaneous mutations in this disorder is high or that the etiology of many cases is not genetic. Among the familial cases, IHH can be inherited as an autosomal dominant, autosomal recessive, or X-linked trait (18). Unique genetic mechanisms have been described for both KS and IHH (67-70). However, some probands with KS have family members with hypogonadism but normal olfaction (Fig. 4) (28, 71, 72). This variable expressivity suggests that it may be more useful to view patients with isolated GnRH deficiency as forming a single large diagnostic spectrum as opposed to fitting in to discrete diagnostic subsets.

Figure 4. A family with GnRH deficiency and multiple affected members in two generations. The pedigree is compatible with autosomal dominant transmission with incomplete penetrance. Note the presence of GnRH deficiency both with and without anosmia in the same family.

X-linked Genes

Kallmann Syndrome Gene (KAL)

Mutations in the KAL-1 gene were the first characterized genetic defect reported to cause GnRH deficiency (73). Anosmin, the protein encoded by the KAL gene, shares homology with molecules involved in neural development and contains 4 contiguous fibronectin type III repeats found in neural cell adhesion molecules (74). Confirmation of the causative role of the KAL gene in KS was provided by a study of a KS fetus with a deletion from Xp22.31 to Xpter, i.e. including the entire KAL gene. Histologic examination of the brain of this fetus demonstrated that migration of the GnRH and olfactory neurons was arrested just below the telencephalon at the cribriform plate (75). Therefore, mutations in the KAL gene appear to cause premature termination of migration of both the olfactory and GnRH neurons to the brain resulting in anosmia and IHH. This profound defect of GnRH neuronal migration results in the complete failure of activation of the HPG axis in patients with X-linked KS (X-KS). Indeed, X-KS patients present with the most severe form of IHH characterized by a complete lack of sexual development, an apulsatile pattern of GnRH-induced LH secretion, a high incidence of cryptorchidism and microphallus, low inhibin B levels, and histologically immature testes (11).

Mutations in the KAL gene are distributed throughout the gene, although most point mutations cluster in the four fibronectin type III repeat domains (76-78). Most X-KS mutations cause alteration of splicing, frameshift or stop codons and result in synthesis of a truncated anosmin protein (79). Missense mutations have rarely been described (79, 80). There appears to be some variability in the phenotypic expression of mutations both within and between families (81, 82). Finally, no correlation has been demonstrated between phenotype and location of the mutation described (79).

DAX-1 Gene

Cases of X-linked adrenal hypoplasia congenita (AHC) accompanied by IHH stem from defects in the DAX-1 gene (Xp21) which encodes a nuclear hormone receptor with a novel DNA-binding domain (83). The AHC phenotype is also marked by clinical heterogeneity. At one end of the spectrum are patients with IHH and normal adrenal function (84), while others have the complete syndrome of adrenal insufficiency in childhood and subsequent hypogonadotropism at puberty (85). The response of patients with AHC to pulsatile GnRH therapy is variable suggesting that certain subsets of these patients are likely to have distinct hypothalamic vs. pituitary defects (86). The failure of hCG to induce normal spermatogenesis points to the presence of a concomitant testicular defect (86-88).

Autosomal Genes

Despite the biology revealed by the X-linked genes, several lines of evidence suggest that autosomal genes account for the majority of familial cases. Of 36 familial cases with GnRH deficiency studied at Massachusetts General Hospital, only 21% could be attributed to X-linkage (20). When the analysis was extended to include surrogate markers of IHH (isolated congenital anosmia and delayed puberty), the X-linked pedigrees comprised only 11%, autosomal recessive 25% and autosomal dominant 64%. These data suggest that in familial cases, the X-linked form of GnRH deficiency is the least common.

GnRH Gene

While the most obvious autosomal candidate gene for GnRH deficiency is the GnRH gene itself, located at 8p21-8p11.2, no deletions, rearrangements, or point mutations in the GnRH gene have been described in the human (69, 89, 90). This is in contrast to the hypogonadal (hpg) mouse in which HH is linked to an autosomal recessive mutation in the GnRH gene and in which gene therapy can completely reverse the hypogonadal phenotype and restore GnRH expression (91).

GnRH-R Gene

Defects in the GnRH-R have recently emerged as the first autosomal cause of IHH (67, 92-96). The gene for the GnRH-R was recently cloned and its product is a G-protein-coupled receptor with seven transmembrane segments (97, 98). Activation of the GnRH-R results in increased activity of phospholipase C and mobilization of intracellular calcium by means of the Gq/G11 group of G proteins. While patients with a GnRH-R mutation were a priori anticipated to manifest complete hypogonadism and unresponsiveness to GnRH stimulation, milder variants of GnRH-R mutations have been described in which GnRH responsiveness is partially maintained. De Roux described the first kindred with a partially inactivating mutation in the GnRH-R (67). The affected male had some testicular growth (8 mL testes), detectable gonadotropins, and a normal response to a single pharmacologic dose of GnRH. Following this initial report, several additional GnRH-R mutations have been described including mutations in the transmembrane domains, which significantly impair GnRH binding and/or signaling (93, 95, 99). These variable genotypes result in a wide phenotypic spectrum ranging from the fertile eunuch syndrome to partial IHH (25, 67, 92) to the most complete form of GnRH deficiency characterized by cryptorchidism, microphallus, undetectable gonadotropins, and absence of pubertal development (93, 95, 99).

The study of 2 patients affected by different contiguous gene syndromes, both of which included KS, led to the recent discovery of a new autosomal dominant gene for KS (100). The presence of overlapping deletions at 8p12-p11 prompted analysis of the gene encoding fibroblast growth factor receptor-1 (FGFR1) in this critical interval. Subsequent determination of the nucleotide sequence of FGFR1 in 129 unrelated individuals with KS led to the identification of heterozygous mutations in 4 familial and 8 sporadic cases consistent with an autosomal dominant mode of inheritance. In addition, one KS patient born to consanguineous parents who had a severe phenotype characterized by cleft palate, agenesis of the corpus callosum, unilateral hearing loss, and fusion of the fourth and fifth metacarpal bones, was found to be homozygous for a missense mutation. Moreover, cleft palate/lip and dental agenesis, two anomalies that are occasionally associated with KS, were noted in several patients with mutations in the FGFR1 gene. The occurrence in one family of synkinesia, a phenotype previously considered to be specific for the X-linked form of KS, indicates that this neurological abnormality can occur also with autosomal transmission.

Linkage analysis of a large consanguineous family, which included 5 siblings with hypogonadotropic hypogonadism, resulted in the identification of a novel locus for autosomal recessive IHH on 19p13 (101). Sequencing of several genes located within this region showed that all affected siblings had a homozygous 155-bp deletion in GPR54, a G protein-coupled receptor gene, while the deletion was absent or present on only 1 allele in unaffected family members (101).

Simultaneously, the role of GPR54 as a regulator of puberty was confirmed by our group, as a homozygous mutation (leu148ser) in the GPR54 gene was identified in six of 19 offspring of a large inbred Saudi Arabian family (102). All subjects found to have a homozygous mutation of GPR54 met the criteria for IHH. Subsequently, analysis of 63 unrelated patients with normosmic IHH and 20 patients with KS revealed a compound heterozygous mutation in the GPR54 gene in an African American male with IHH (102).

Genetic Defects of the Gonadotropins

Mutation in the Gene for LH-beta subunit

There is a single case report of a male who failed to go through puberty and was found to have a homozygous mutation in amino acid 54 of the LH-beta subunit, eliminating the ability of LH to bind to its receptor (103). The mutant hormone had no biologic activity but had normal immunoreactivity. Testosterone levels were reduced in association with elevated LH and FSH levels. This clinical picture could easily be confused with primary hypogonadism had the patient not been shown to have a normal serum testosterone response to hCG administration.

Mutations in the Gene for FSH-beta subunit

The first reported male with an FSH-beta gene mutation (Cys82Arg missense) appeared normally virilized and had normal LH and testosterone levels, but small testes, azoospermia, and a high FSH level (104). The second reported case of a man with an FSH-b gene mutation (Val61X) had very small testes (1 and 2 mL) and an unexpected absence of pubertal development; he was found to harbor an additional defect in Leydig cell function as evidenced by high LH and low testosterone levels (105).

Hypogonadotropic hypogonadism associated with other pituitary hormone deficiencies

Combined pituitary hormone deficiency has been linked with rare abnormalities in genes encoding transcription factors necessary for pituitary development. Mutations in the gene, PROP-1 (Prophet of Pit-1) appear to be the most common cause of both familial and sporadic congenital combined pituitary hormone deficiency. Several different mutations have been recognized and all show recessive inheritance. The hormonal phenotype involves deficiencies of LH, FSH, GH, PRL, and TSH. Hypogonadism is a striking feature of patients with PROP-1 mutations, but there is variability in its clinical and hormonal expression. Most affected subjects fail to enter puberty and show consistently low LH and FSH responses to GnRH stimulation (106). However, there is a report of two sibships with a homozygous mutation, Arg120Cys in PROP-1 in which the affected children experienced reversal of puberty following its initiation (107). The same sequence of events was observed in several children with mutations causing a complete loss of function raising the questions of whether this a model of acquired rather than congenital gonadotropin deficiency (108).

EVALUATION

Hypogonadism is defined as a defect in one of the two major functions of the testes i.e. production of testosterone and spermatogenesis. The presence of hypogonadism can reflect disorders intrinsic to the testes (primary or hypergonadotropic hypogonadism) or disorders of the pituitary or hypothalamus (secondary or hypogonadotropic hypogonadism). These two entities can be distinguished by measuring serum LH and FSH concentrations (Fig. 5). Primary hypogonadism is characterized by a low serum testosterone level and oligo- or azoospermia in the presence of elevated serum LH and FSH concentrations. In contrast, secondary hypogonadism is diagnosed in the setting of a low testosterone level and sperm count in association with low or inappropriately normal serum LH and FSH concentrations.

It is best to measure testosterone in a morning sample given the normal diurnal rhythm of testosterone secretion. Repeat measurements should be performed if the initial reading is low. In secondary hypogonadism, measuring the serum LH response to a single bolus of exogenous GnRH is not helpful in distinguishing pituitary from hypothalamic disease, because a subnormal response may occur in both settings. Patients with IHH due to congenital absence of hypothalamic GnRH secretion may have had no prior exposure to GnRH; in this setting, repeated administration of GnRH is needed to prime the gonadotropes to elicit a gonadotropin response. Characterization of the pulsatile pattern of LH secretion with frequent blood sampling is useful in refining the diagnosis in a research setting, but is not practical in the clinical setting.

A semen analysis should be obtained to assess sperm production and the count, motility, and morphology determined. Based on WHO criteria, the parameters for a normal semen analysis are a sperm count of >20 million sperm/mL with motility of >50% and normal morphology in >30% (109). Two normal semen analyses are sufficient to indicate that the count and motility are normal.

Finally, in the case of secondary hypogonadism, it is critical to assess the rest of the pituitary axis (see chapter 6), including a prolactin level to ensure that the defect is isolated to the HPG axis. A ferritin level should be measured to exclude hemochromatosis. Patients with HH typically undergo adrenarche at a normal age and should therefore have normal adult male levels of DHEAS. Radiographic evaluation should include a bone age determination, MRI of the pituitary and hypothalamic area and a DEXA scan to assess bone mineral density.

TREATMENT

Choosing the most appropriate treatment for men with HH should be based upon an informed discussion between the patient and his physician. Androgen therapy, whether by exogenous testosterone replacement or induction of endogenous testosterone production by hCG is needed in all HH patients. Androgens play a number of important physiologic roles in the human and are required not only for virilization and normal sexual function but also for maintenance of both muscle and bone mass, as well as normal mood and cognition.

While testosterone is the primary treatment modality used to induce and maintain secondary sexual characteristics and sexual function in men with hypogonadism, treatment with testosterone does not restore fertility. Therefore in patients in whom fertility is the treatment goal, induction of gonadotropin secretion by GnRH or treatment with exogenous gonadotropins is necessary.

Testosterone Substitution

Testosterone is currently available in a variety of formulations for clinical use including injectable esters as well as transdermal patch and gel preparations, each with its own unique pharmacokinetic profile. This topic is discussed in detail in chapter 6, and 7, and erectile dysfunction is discussed in Chapter 8.

In patients who desire fertility, the options to induce spermatogenesis include exogenous gonadotropins or pulsatile GnRH. In HH, the origin of the disease influences the choice of treatment to achieve fertility. GnRH substitution is more effective for hypothalamic than pituitary disorders. However, depending on the number of functioning gonadotropes remaining, GnRH may also be an effective therapy for patients with hypopituitarism. Administration of exogenous gonadotropins is suitable for patients with both pituitary and hypothalamic disorders. Conventional therapy uses hCG as an LH substitute in conjunction with FSH in the form of either human menopausal gonadotropins, highly purified urinary FSH preparations or recombinant FSH formulations. A recombinant form of LH is currently being evaluated.

The role of FSH in the initiation and maintenance of spermatogenesis in the human is controversial (105). There is discordance between the phenotype of men with mutations in the FSH receptor (FSH-R) gene who are oligospermic (110) and those with mutations of the FSH-beta subunit who are azoospermic (104, 105). If one were to assume that the FSH-R mutations were able to abolish FSH action completely, this report suggests that LH and testosterone alone are sufficient to induce spermatogenesis given that all five reported cases had sperm in the ejaculate and two had fathered children (110). However, the demonstration that the two patients with a mutation in the FSH-� subunit were azoospermic (104, 105) suggests that the degree of inactivation of the FSH-R may have been incomplete. However, interpretation of this report is confounded by the fact that one of these patients also had hypogonadal testosterone levels so that the azoospermia in this case could not be attributed to lack of FSH alone (105).

Several other lines of evidence support a role for FSH in the maintenance of spermatogenesis. In contraceptive trials using testosterone esters, azoospermia is achieved only in those patients whose serum FSH levels are rendered undetectable (111, 112). Similarly, there is a case report of a patient with a history of a hypophysectomy for a pituitary adenoma who was found to be fertile when treated with testosterone alone due a concomitant activating mutation of the FSH-R (113). However, in a subset of patients with HH and larger testicular size, spermatogenesis can be stimulated with hCG alone (114). This group largely comprises men with the fertile eunuch syndrome (26, 27), who may well have sufficient endogenous FSH secretion to sustain normal spermatogenesis with hCG alone.

In addition to FSH, adequate intra-testicular testosterone concentrations are also essential for normal spermatogenesis as illustrated by a study of hypogonadal men in which spermatogenesis could be induced by the combination of hCG and hMG but not by a regimen comprising purified FSH and testosterone (115). In summary, qualitatively and quantitatively normal spermatogenesis is best maintained in the presence of both FSH and LH-induced T secretion.

The typical gonadotropin regimen for induction of spermatogenesis in men comprises hCG in combination with FSH. Purified hCG is an effective substitute for LH given the structural homology between these 2 hormones which act through the same Leydig cell receptor. While a variety of FSH formulations are now available in different countries, there is little to choose between them in terms of therapeutic efficacy. Traditionally, FSH has been administered in the form of human menopausal gonadotropins (hMG) derived from the urine of postmenopausal women. Although hMG has both FSH and LH activity, FSH activity predominates and LH activity is so low that combined administration with hCG is necessary to achieve fertility (116). Subsequently, highly purified urinary FSH preparations were developed, which have enhanced specific activity in comparison to hMG (10,000 IU/mg of protein vs 150 IU/mg of protein for hMG). In the early 1990s, recombinant FSH (r-hFSH) formulations were developed, which have greater purity and specific activity than any of the urinary preparations and no intrinsic LH activity (117, 118). r-hFSH is produced in genetically engineered Chinese hamster ovary cells, in which the genes encoding the alpha and beta subunits have been introduced using recombinant DNA technology (117). Pharmacokinetic studies of r-hFSH indicate a half-life of 48 � 5 h and a dose-dependant increase in the serum level of FSH.

In our experience, the subcutaneous route of administration is as effective as the intramuscular route for both gonadotropins and significantly increases patient compliance. Therapy is typically initiated with hCG alone at a dose of 1,000 IU on alternate days and the dose titrated based on trough testosterone levels and testicular growth. In the majority of patients with larger testes at baseline, spermatogenesis can be initiated with hCG alone most likely due to residual FSH secretion (114, 119, 120). Once there is a plateau in the response to hCG, therapy with FSH (in one of the three forms described above) should be added at a dose of 75 IU alternate days. Continuation of this combined regimen for 12-24 months induces testicular growth in almost all patients, spermatogenesis in a large proportion (116, 121-123) and pregnancy rates in the range of 50 to 80% (116). Factors predictive of better outcome include larger baseline testicular size, and absence of cryptorchidism (116, 124). Gynecomastia is the most common side effect of gonadotropin therapy and is seen in up to one third of patients. The mechanism underlying the development of gynecomastia in this setting appears to be excessive secretion of estrogen in response to hCG stimulation of the Leydig cells. This undesirable side effect can be prevented by using the lowest dose of hCG capable of maintaining serum testosterone levels towards the lower end of the normal range (116, 124).

The majority of HH patients treated with gonadotropins have sperm counts that remain below the normal range. However, failure to achieve normal sperm counts does not preclude fertility. Indeed, in 1 study of IHH men treated with gonadotropins, the median sperm concentration at conception was reported to be 5 million/mL (114). While spermatogenesis can be initiated even in patients with very small testes i.e.<3 mL (17, 114), a longer duration of therapy is typically required and it may take up to 24 months for spermatogenesis to be induced. Accordingly when discussing the issue of fertility with patients, we recommend that they start treatment 6 to 12 months prior to the time at which fertility is desired. Once pregnancy is achieved, we advise continuing therapy until at least the second trimester. If the couple plan to have another child in the near future, therapy with hCG alone should be continued. However, if a long interval is expected to elapse before the next pregnancy, it may be more convenient for the patient to resume testosterone therapy. Patients should also be given the option of storing sperm for subsequent use in intra-uterine insemination or intracytoplasmic sperm injection. In patients in whom the combination of hCG and FSH is required to induce spermatogenesis initially, treatment with hCG alone may be sufficient for subsequent pregnancies due to larger testicular size.

In patients with panhypopituitarism who fail to respond to gonadotropin therapy, the addition of recombinant growth hormone (rGH) therapy should be considered. It is thought that a direct effect of growth hormone on Leydig cells may play a role in the delayed puberty encountered commonly in patients with isolated growth hormone deficiency (125, 126). However, available data on the role of rGH in inducing spermatogenesis in men with hypopituitarism are derived from small, non-randomized studies and give conflicting results; the results of larger, randomized studies are awaited before a definitive decision about the benefit of rGH in this setting can be reached.

2. Pulsatile GnRH Therapy

The alternative to gonadotropin therapy is pulsatile administration of GnRH, which may be administered by a programmable, portable mini-infusion pump (Fig. 6). While intravenous administration produces the most physiologic GnRH pulse contour and ensuing LH response (127), the subcutaneous route is clearly more practical for the longterm treatment required to stimulate spermatogenesis. Based on our normative data (7), the frequency of GnRH administration that we employ is every 2 hours. The dose of GnRH is titrated for each individual to ensure normalization of testosterone, LH and FSH and varies from 25 to 600 ng/kg per bolus. Patients on longterm therapy are monitored with serum testosterone and gonadotropin levels at monthly intervals. Once testicular volume reaches 8 mL, regular semen analyses are obtained. The majority of patients require treatment for at least 2 years to maximize testicular growth and achieve spermatogenesis, although the time taken to reach these endpoints tends to be shorter in those with a larger initial gonadal size. In our experience, pulsatile GnRH is successful in inducing spermatogenesis in the vast majority of patients with HH, the exceptions tending to be those with a history of bilateral cryptorchidism.

Reversal of Idiopathic Hypogonadotropic Hypogonadism.: Sustained reversal of normosmic idiopathic hypogonadotropic hypogonadism and the Kallmann syndrome was noted after discontinuation of treatment in about 10% of patients with either absent or partial puberty. Therefore, brief discontinuation of hormonal therapy to assess reversibility of hypogonadotropic hypogonadism is reasonable.( Raivio T, Falardeau J, Dwyer A, Quinton R, Hayes FJ, Hughes VA, Cole LW, Pearce SH, Lee H, Boepple P, Crowley WF Jr, Pitteloud N.Reversal of Idiopathic Hypogonadotropic Hypogonadism N Engl J Med. 2007 Aug 30;357(9):863-873)

Figure 6. LH and testosterone (T) at baseline and in response to GnRH therapy in a man with complete GnRH deficiency. Baseline LH secretion pattern is apulsatile with a hypogonadal T level. Exogenous pulsatile GnRH therapy administered every 2 h stimulates normal LH pulses which in turn induce a normal serum T level.

If pulsatile GnRH treatment fails, a mutation of the GnRH-R gene should be considered and genetic testing arranged if available (92). A second cause for failure of pulsatile GnRH treatment is the appearance of anti-GnRH antibodies, which typically occur in the setting of erratic compliance and are associated with a progressive decrease in T and gonadotropins levels.
However, the use of pulsatile GnRH therapy for induction of spermatogenesis is not approved by the Food and Drug Administration in the United States and is thus confined to specialist centers. In our NIH-funded research program at Massachusetts General Hospital, we are able to provide IHH patients with pulsatile GnRH therapy free of charge in return for their participation in some of our research studies. Part of our research program is focussed on identifying the genes that cause GnRH deficiency in the human, as a result of which we are able to offer patients free genetic testing.

Both exogenous gonadotropins and pulsatile GnRH are very effective in stimulating spermatogenesis. Most studies have no shown no advantage of either therapy in terms of testicular growth, onset of spermatogenesis, final sperm counts or pregnancy rates (116, 124). However, there are some data to suggest that testicular growth is greater and the time taken to achieve spermatogenesis is shorter in patients treated with GnRH (128).

Assisted Reproductive Technology

As a result of a number of exciting advances in the area of assisted reproductive technology (ART), couples who would previously have been offered donor insemination or adoption are now achieving pregnancies despite persistent severe impairments in sperm count and/or quality after hormonal therapy. While male factor infertility was initially considered a contraindication to in vitro fertilization (IVF), IVF is now considered as an acceptable treatment option for the infertile male. However, for men with severe male factor infertility, IVF is often unsuccessful and more sophisticates techniques to assist fertilization such as intracytoplasmic sperm injection (ICSI) are required (129). The success rates of ICSI are high with fertilization rates of 50-60% and pregnancy rates of ~30% per cycle. If no sperm are present in the ejaculate, sperm retrieved either from epididymal aspiration or from testicular aspiration/biopsy can be used successfully for ICSI. The incidence of congenital malformations does not appear to be increased. However, a slight increase in chromosomal aberrations, especially of the sex chromosomes, has been observed (130). The increase in chromosomal abnormalities underscores the importance of proper genetic counselling of couples contemplating ICSI.


	HYPOGONADOTROPIC HYPOGONADISM (HH) AND GONADOTROPIN THERAPY Chapter 5 - Frances J Hayes MD, and Nelly Pitteloud MD February 6, 2004	Index Contributors Search