Gynecomastia, breast development in males, can occur normally during three phases of life. The first occurs shortly after birth in both males and females. This is caused by the high levels of estradiol and progesterone produced by the mother during pregnancy, which stimulates newborn breast tissue. It can persist for several weeks after birth and can cause mild breast discharge called "witch's milk" (47). Puberty marks the second situation in which gynecomastia can occur physiologically. In fact, up to 60% of boys have detectable gynecomastia by age 14. Although it is mostly bilateral, it can occur unilaterally, and usually resolves within 3 years of onset (47).

Interestingly, in early puberty, the pituitary gland releases gonadotropins in order to stimulate testicular production of testosterone mostly at nighttime. Estrogens, however, rise throughout the entire day. Some studies have shown that a decreased androgen to estrogen ratio exists in boys with pubertal gynecomastia when compared with boys who do not develop gynecomastia (37). Furthermore, another study showed increased aromatase activity in the skin fibroblasts of boys with gynecomastia. Thus, the mechanism by which pubertal gynecomastia occurs may be due to either decreased production of androgens or increased aromatization of circulating androgens, thus increasing the estrogen to androgen ratio (32).

The third age range in which gynecomastia is frequently seen is during older age (>60 years). Although the exact mechanisms by which this can occur have not been fully elucidated, evidence suggests that it may result from increased peripheral aromatase activity secondary to the increase in total body fat, coupled with mild hypogonadism associated with aging. For instance, investigators have shown increased urinary estrogen levels in obese individuals, and have demonstrated aromatase expression in adipose tissue (39). Thus, like the gynecomastia of obesity, the gynecomastia of aging may partly result from increased aromatase activity, causing increased conversion of androgens to estrogens(8). Moreover, not only does total body fat increase with age, but there may be an increase in aromatase activity in the adipose tissue already present, increasing circulating estrogens even further. Lastly, SHBG increases with age in men. Since SHBG binds estrogen with less affinity than testosterone, the bioavailable estradiol to bioavailable testosterone ratio may increase in the obese older male.