Since the development of breast tissue in males occurs in an analogous manner to that in females, the same hormones that affect female breast tissue can cause gynecomastia. The testes secrete only 6-10 mg of estradiol and 2.5 mg of estrone per day. Since this only comprises a small fraction of estrogens in circulation (i.e. 15% of estradiol and 5% of estrone), the remainder of estrogen in males is derived from the extraglandular aromatization of testosterone and androstenedione to estradiol and estrone (33). Thus, any cause of estrogen excess from overproduction to peripheral aromatization of androgens can initiate the cascade to breast development.

Testicular tumors can lead to increased blood estrogen levels by: estrogen overproduction; androgen overproduction with aromatization in the periphery to estrogens; and by ectopic secretion of gonadotropins which stimulate otherwise normal Leydig cells. Tumors causing an overproduction of estrogen represent an unusual but important cause of estrogen excess. Examples of estrogen-secreting tumors include: Leydig cell tumors, Sertoli cell tumors, granulosa cell tumors and adrenal tumors.

Interstitial cell tumors, or Leydig cell tumors constitute 1%-3% of all testis tumors. Usually, they occur in men between the ages of 20 and 60, although up to 25% of them occur prepubertally. In prepubertal cases, isosexual precocity, rapid somatic growth, and increased bone age with elevated serum testosterone and urinary 17-ketosteroid levels are the presenting features. In adults, elevated estrogen levels coupled with a palpable testicular mass and gynecomastia suggests a testicular tumor. Though mostly benign, Leydig cell tumors may be malignant and metastasize to lung, liver, and retroperitoneal lymph nodes (42, 19).

Sertoli cell tumors comprise less than 1% of all testicular tumors and occur at all ages, but one third have occurred in patients less than 13 years, usually in boys under 6 months of age. Although they arise in young boys, they usually do not produce endocrine effects in children. Again, the majority are benign, but up to 10% are malignant. Gynecomastia occurs in one third of cases of Sertoli cell tumors, presumably due to increased estrogen production (42).

Granulosa cell tumors, occurring very rarely in the testes, can also overproduce estrogen. In fact, only eleven cases have been reported with gynecomastia as a presenting feature in half of them (34).

Germ cell tumors are the most common cancer in males between the ages of 15 and 35. They are divided into seminomatous and nonseminomatous subtypes and include embryonal carcinoma, yolk sac carcinoma, choriocarcinoma and teratomas. Elevated alpha fetoprotein (AFP) and  HCG function as reliable markers in some tumors. As a result of the increased  HCG, acting analogously to LH to stimulate the Leydig cell LH receptor, testicular estrogen production is also increased, which, in turn, can cause gynecomastia. Although germ cell tumors generally arise in the testes, they can also originate extra-gonadally, specifically in the mediastinum. These extragonadal tumors also possess the capability of producing  HCG, but they must be differentiated from a multitude of other tumors such as large cell carcinomas of the lung which can synthesize ectopic  HCG (38).

Some neoplasms that overproduce estrogens also possess aromatase overactivity. Sertoli Cell tumors in boys with Peutz-Jegher syndrome, an autosomal dominant disease characterized by pigmented macules on the lips, gastrointestinal polyposis and hormonally active tumors in males and females, for instance, have repeatedly demonstrated aromatase overactivity, resulting in gynecomastia, rapid growth and advanced bone age as presenting features (23, 56, 14). Feminizing Sertoli cell tumors with increased aromatase activity can also be seen in the Carney complex, an autosomal dominant disease characterized by cardiac myxomas, cutaneous pigmentation, adrenal nodules and hypercortisolism. Other than sex-cord tumors, fibrolamellar hepatocellular carcinoma has also been shown to possess ectopic aromatase activity, causing severe gynecomastia in a 17-year-old boy (2). Furthermore, adrenal tumors can secrete excess dehydroepiandrosterone (DHEA), DHEA-sulfate (DHEAS) and androstenedione that can then be aromatized peripherally to estradiol.