Stimulation of testosterone secretion by hCG is a useful diagnostic test for examining Leydig cell function in prepubertal boys who secrete little or no endogenous gonadotropin. A variety of clinical protocols have been used. In one study, serum testosterone levels rose to >300 ng/dL in prepubertal boys given seven intramuscular injections of hCG, 1500 IU, every other day.89 Testosterone levels usually fail to increase in plasma in boys with congenital bilateral anorchia, but increase somewhat in boys with bilateral intraabdominal testes.90 Based on the results of this test, as well as the level of müllerian inhibitory substance, the latter patients will undergo laparoscopy followed by orchidopexy or orchiectomy91 (see Chap. 93). The change in the circulating level of testosterone and its precursor steroids after hCG stimulation has been used to distinguish between boys with primary hypogonadism, defects in testosterone biosynthesis, and boys with androgen-insensitivity syndromes.92

Adult men with primary testicular failure have elevated serum LH concentrations. The administration of hCG to these men predictably increases serum testosterone levels less than in normal men.93 In gonadotropin-deficient men, the short-term administration of hCG will also produce a blunted testosterone response because Leydig cells have been exposed to subnormal LH stimulation. Overall, little clinically useful information is gained from hCG testing in either group of adult men.

The estrogen antagonists, clomiphene and tamoxifen, have been used to assess the integrity of the hypothalamic–pituitary–testicular unit in adult men with borderline low testosterone levels and possible hypogonadotropic hypogonadism. This clinical situation often is encountered in obese94 and in elderly men.95 Endogenous estrogens suppress gonadotropin secretion in normal men, and blocking this suppression increases gonadotropin concentrations. Dosages of 100 to 200 mg per day of clomiphene for 7 days produce a mean twofold increase in serum LH, FSH, and testosterone levels in normal men.96 A normal response implies the functional integrity of GnRH-LH pathways, but responses among subjects are variable, and the test is probably not useful clinically.

GnRH is used as a research tool to examine the ability of the gonadotrope cells to release LH and FSH in response to their hypophysiotropic stimulus (see Chap. 16). The GnRH test was introduced as a method for diagnosing hypogonadism and for discriminating hypothalamic from pituitary disorders. In normal adult men given 100 μg GnRH intravenously, serum LH levels increase three- to- sixfold, and serum FSH levels rise by 50%. Generally, the total and incremental release of LH and FSH after GnRH administration is proportional to the basal hormone level,97 although exceptions occur. In adult men with either hypothalamic or pituitary disease, the release of gonadotropins after GnRH stimulation usually is reduced, but may be normal. Overall, in evaluating gonadotropin-deficient men, the GnRH test provides little information beyond that of the basal testosterone, LH, and FSH levels; therefore, it is not recommended. Furthermore, the presence of a hypothalamic or pituitary mass is most directly demonstrated by magnetic resonance imaging. Treatment with GnRH will increase LH and FSH secretion in GnRH-deficient men, but is less effective in men with hypogonadotropic hypogonadism due to GnRH-receptor mutations.

Peak serum gonadotropin levels after GnRH stimulation are increased in patients with primary testicular failure. Subtle testicular dysfunction may produce an exaggerated response even with normal basal LH and FSH levels.98 The LH response to a subcutaneous dose of a potent GnRH agonist in boys with delayed puberty may exceed that of patients with congenital hypogonadotropic hypogonadism, and may be useful in distinguishing between these two patient groups,99 although not all studies are in agreement.

The semen analysis is a simple, inexpensive, and useful test to evaluate testicular function. Complete details of the laboratory
methods for semen analysis are available in a comprehensive manual published by the World Health Organization100 (Table 114-4). There is an effect of duration of abstinence on semen volume and sperm output. Accordingly, a fixed abstinence interval of 2 to 3 days is suggested to standardize results. There is also day-to-day variability in sperm output. Therefore, if the initial sample is not entirely normal, three samples may be obtained during one sperm cycle (72 days). Since acute and chronic illness and surgery suppress gonadal function, semen samples should be evaluated at a time when the patient has been in good health.