Basic endocrine evaluation includes measurement of serum testosterone (T) and follicle-stimulating hormone (FSH). Testosterone is necessary for the development and maintenance of secondary sexual characteristics and libido as well as initiation and maintenance of spermatogenesis. Serum FSH crudely reflects the status of the seminiferous epithelium. Elevated serum FSH results from impaired secretion of inhibin, a Sertoli cell product that normal feeds back at the pituitary and hypothalamus to turn off FSH secretion and suggests abnormalities in the seminiferous epithelium and subsequently spermatogenesis. An FSH level greater than two to three times the upper limits of normal suggests severely impaired seminiferous tubule , but may still be treatable. Luteinizing hormone (LH) is stimulatory to the Leydig cells and hence T production. Isolated LH abnormalities are very rare. LH levels need be obtained only in men with abnormal T levels.
Low levels of FSH, LH, and T are diagnostic of hypogonadotropic hypogonadism. These men have a delay or failure in the onset of puberty and therefore poorly developed secondary sexual characteristics and small firm testes. Testosterone replacement will masculinize these men but testicular growth and the initiation of spermatogenesis requires gonadotropin replacement. Hypogonadotropic hypogonadism is usually due to a pituitary tumor, with the most common pituitary lesion being a benign prolactinoma. These are usually associated with a decreased libido, an elevated serum prolactin level, and decreased serum T and LH levels. Both macro and microadenomas are often best treated with bromocriptine. Serum estrogens, prolactin, and adrenal steroids are only measured if clinically indicated (low serum T, decreased libido, gynecomastia, or a history of precocious puberty).