The onset of pubertal gynecomastia is during Tanner stage II-IV. Onset after puberty is complete (Tanner stage V) is always pathologic. The boy often notes breast development early in puberty and is concerned that he may be abnormal, as little information is shared about this phenomenon between males. All hormones, including FSH, LH, prolactin, testosterone, and estradiol are normal. The breast development rarely persists more than two years. 

Unfortunately, breast development may also be a sign of a pathologic process. A good history and physical examination are necessary for males with breast development, especially those with large (>4 cm diameter) or female appearing breasts. The history should include details concerning onset of puberty, drug use and systemic illness. The physical should include examination of visual fields, thyroid, breast, abdomen and the genitals. Usually no laboratory studies are necessary. However, if something of concern is found in the history or physical examination, laboratory studies may include FSH, LH, tests of thyroid and liver function, androgens, estradiol, MRI of the sella, chest x-ray for evidence of tumor, karyotype, and tumor markers such as human chorionic gonadotropin (HCG) and alpha-fetoprotein (AFP). 

The differential diagnosis includes:

• Familial macromastia (e.g., autosomal dominant where males grow female type breasts)
• Drugs
• Steroids- anabolic, estrogen
• Antibiotics- isoniazid, ketoconazole
• Illicit- marijuana, amphetamines, opiates
• Psychotropic agents- tricyclics, phenothiazines
• Miscellaneous- cimetidine, digoxin, phenytoin
• Hypergonadotropic hypogonadism
• Klinefelter's syndrome (47, XXY)
• Reifenstein's syndrome (partial androgen insensitivity)
• Thyroid dysfunction
• Tumors
• Testicular
• Bronchogenic carcinoma (mainly in adults)
• Pituitary - prolactinoma
• Adrenal
• Hodgkin's disease
• Renal failure
• Liver disease
• Recovery from malnutrition
• Pseudogynecomastia (e.g., adipose tissue in the area of the breast)