TAKE HOME POINTS

1-The differential diagnosis of HH includes structural, functional and genetic abnormalities affecting the hypothalamic-pituitary-gonadal axis.

2-While our understanding of the genetic basis of HH has expanded considerably in the past decade, mutations have only been identified in approximately 40% of patients to date.

3-The traditional view that congenital HH is a simple monogenic disorder has been challenged by the demonstration of oligogenicity, which helps explain the phenotypic variability underlying this condition.

4-HH is a treatable cause of male infertility with larger testicular size and higher baseline inhibin B levels predicting a favorable response to gonadotropin or GnRH therapy, while negative prognostic indicators include cryptorchidism and possibly prior androgen use.

5-While lifelong androgen replacement is necessary in most men with HH, a brief discontinuation of hormonal therapy should be considered in all cases given that reversibility has been demonstrated in up to 10% of cases.

DEFINITIONS

Pulsatile secretion of gonadotropin-releasing hormone (GnRH) from the hypothalamus is required for both the initiation and maintenance of the reproductive axis in the human. Pulsatile GnRH stimulates biosynthesis of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) that in turn initiate both intra-gonadal testosterone production and spermatogenesis as well as systemic testosterone secretion and virilization. Failure of this episodic GnRH secretion or action, or disruption of gonadotropin secretion, results in the clinical syndrome of hypogonadotropic hypogonadism (HH). Disorders causing HH are differentiated from primary testicular disease by the demonstration of low/normal gonadotropin levels in the setting of low testosterone concentrations and sperm counts (Fig. 1). Congenital abnormalities leading to HH are rare but well described and are usually the consequence of deficient GnRH secretion occurring either in isolation (normosmic congenital hypogonadotropic hypogonadism (nCHH)), or in association with anosmia (Kallmann syndrome (KS)). A growing number of loci have been associated with congenital GnRH deficiency and mutations in the GnRH receptor, as well as both LH-b and FSH-b subunits, have been reported. Acquired causes of HH are more common and can be due to any disorder that affects the hypothalamic-pituitary axis.

Fig. 1Schematic of the hypothalamic-pituitary-gonadal axis in a normal adult male and in the setting of primary and secondary hypogonadism.

NORMAL GnRH SECRETION ACROSS DEVELOPMENT IN THE MALE

In the human, the pattern of GnRH-induced gonadotropin secretion is constantly changing during sexual development. Therefore, the establishment of a robust normative database is critical for understanding pathologic states like HH.

Adulthood

During adulthood, gonadotropins are secreted in a pulsatile fashion. In the adult male, LH is secreted in pulses approximately every 2 hours (Fig. 2A) (7). However, considerable variability is observed in LH pulse patterns and there is a wide range of testosterone secretory patterns. Indeed, in 15% of normal men whose hypothalamic-pituitary-gonadal (HPG) axis was examined using frequent blood sampling, serum testosterone levels as low as 3.5 nmol/L were recorded (to convert to ng/dL, multiply by 28.6) following long inter-pulse intervals of LH secretion, although mean testosterone levels remained within the normal range (Fig. 3). This within-patient variation must be considered when interpreting single LH and testosterone measurements obtained during the evaluation of a male with suspected hypogonadism. This variability is particularly important in middle-aged and older men as up to 30% men that are found to have a low testosterone concentration, will have a normal level on repeat testing (8).

Fig. 2

Spectrum of GnRH-induced LH secretion in men with GnRH deficiency. LH pulsations are indicated by asterisks. A, Normal adult male pattern of GnRH secretion with high amplitude regular LH pulsations and normal serum testosterone (T), testicular volume (TV) and sperm count; B, Disordered amplitude pattern of GnRH secretion in an CHH male, characterized by low amplitude LH pulsations, low serum T, and azoospermia; C, Sleep-entrained or developmental arrest pattern of LH secretion in an CHH male characterized by relatively low amplitude LH pulsations clustered during the night-time hours analogous to the pattern which normally occurs at puberty. Note that the TV is higher than in the subject with the apulsatile pattern; D, Apulsatile pattern of GnRH secretion in an CHH male with complete absence of endogenous LH pulsations, low serum T, prepubertal TV and azoospermia.

Fig. 3

Frequent blood sampling (q 10 min for 24 h) of serum testosterone (T) and LH in a normal male. Note that a T level of 3.2 nmol/L (to convert to ng/dL, multiply by 28.6) which is in the hypogonadal range was recorded after a long interpulse interval of LH secretion.

CLINICAL PRESENTATION

The clinical features of GnRH and/or gonadotropin deficiency are typically first manifested at puberty, a time when there is normally a marked increase in GnRH secretion. The phenotypic presentation of HH varies with age of onset (congenital vs. acquired), severity (complete vs. partial), and duration (functional vs. permanent).

HH may also be diagnosed in the neonatal period. The typical clinical phenotype is that of a male infant with normal sexual differentiation, cryptorchidism and micropenis in whom gonadotropin and sex steroid levels are inappropriately low given the normal activation of the HPG axis during this period. Early in fetal life, the testosterone production required for full sexual differentiation is thought to be stimulated by maternal hCG alone. However, endogenous secretion of GnRH in the late fetal/early neonatal periods appears necessary for inguino-scrotal descent of the testes and full growth of the external genitalia (9, 10). Accordingly, cryptorchidism and microphallus have been reported in up to 50% of patients with IHH and KS in some small series (11, 12) and may represent surrogate markers of failure of activation of GnRH secretion during the neonatal window.

Most often, the diagnosis of HH is delayed until adolescence, when there is a failure to go through puberty. The clinical presentation includes lack of development of secondary sex characteristics, eunuchoidal body proportions (upper/lower body ratio <1 with an arm span 6 cm > standing height), a high-pitched voice, mild anemia, delayed bone age, and pre-pubertal testes (12). However, the syndrome is clinically heterogeneous in that some patients have evidence of partial spontaneous pubertal development reflected by larger gonadal size despite hypogonadal testosterone levels and inappropriately low gonadotropin levels. Gynecomastia is not a typical feature of GnRH deficiency given the hypogonadotropic state and is most commonly seen in patients treated with gonadotropins (13, 14). HH may also present after completion of puberty resulting in a disruption in reproductive function in adulthood characterized by decreased libido, erectile dysfunction and oligo- or azoospermia (15).

DIFFERENTIAL DIAGNOSIS

Hypogonadotropic hypogonadism may be broadly classified into congenital or acquired disorders (Table 1).

Congenital Hypogonadotropic Hypogonadism

CHH is characterized by an isolated defect in GnRH secretion as evidenced by: i) complete or partial absence of GnRH-induced LH pulsations (Fig. 1) (16, 17); ii) normalization of pituitary-gonadal axis function in response to physiological regimens of exogenous GnRH replacement (16-18); iii) otherwise normal hormonal testing of the anterior pituitary including a normal ferritin level and; iv) normal imaging of the hypothalamic-pituitary region.

CHH was first reported in association with anosmia and termed Kallmann syndrome (19). However, it was subsequently appreciated that several patients with CHH lack evidence of an olfactory defect and thus have a normosmic form of CHH. While the majority of CHH patients present with lack of pubertal development, there is considerable clinical heterogeneity. Depending on the degree of prior spontaneous pubertal development, testicular size in men with CHH may range from prepubertal to near-normal adult testes. In addition to anosmia, a variety of other anomalies have been reported to occur in CHH with an increased frequency in the KS subset including cleft lip and palate, synkinesia, sensorineural deafness, cerebellar ataxia, renal agenesis, digital bony abnormalities and dental agenesis (19-21). Some of these clinical features are highly associated with particular genetic causes of KS so that the clinical phenotype can be utilized to prioritize genetic testing (22).

Given that it is a rare disease, data on the incidence of CHH is limited with estimates varying from 1/10,000 to 1/86,000 (23, 24). Isolated GnRH deficiency occurs more commonly in men than in women. Based on our review of 250 consecutive cases seen at the Massachusetts GeneralHospital, the male:female ratio is 4:1.

Acquired Hypogonadotropic Hypogonadism Functional

Functional forms of HH are characterized by a transient defect in GnRH secretion. This type of presentation occurs most commonly in female hypogonadotropic subjects with hypothalamic amenorrhea (HA). In susceptible individuals, HA may be precipitated by factors such as significant weight loss, exercise or stress (32-34). In addition, a genetic predisposition to HA has also been identified with the demonstration of rare variants in genes associated with CHH in some women with this disorder (35). Typically, GnRH secretion will resume after correcting the underlying abnormality and menstruation will be restored. While in women the presence or absence of menses acts as a useful clinical marker of the functioning of the HPG axis, there is no comparable clinical marker in the male. Moderate to severe dietary restriction in otherwise healthy men has been shown to decrease testosterone levels by impairing secretion of GnRH (36, 37). In addition, some (38, 39) but not all (40) studies have shown that strenuous physical exercise may adversely affect testosterone concentrations. However to date, a clinical syndrome of functional GnRH deficiency in men that is analogous to HA has not been definitively established.

Drug-Induced GnRH Deficiency

Use of anabolic steroids may result in a functional form of HH manifested by decreased concentrations of both testosterone and dihydrotestosterone and a marked impairment of spermatogenesis (41, 42). While the suppression of the HPG axis induced by anabolic steroids is reversible, rate of recovery following cessation of steroid use is variable and can range from 4-12 months (41, 43). Chronic treatment with glucocorticoids may also lead to hypogonadism. In one study, 16 men with chronic pulmonary disease who received high-dose glucocorticoids for at least one month had a mean serum testosterone of 6.9 nmol/L, compared to 15.6 nmol/L in 11 men matched for age and disease (44). Given that serum LH levels did not increase in this study, these data suggest a predominantly central mechanism for glucocorticoid-induced hypogonadism. Chronic use of oral and intrathecal narcotic analgesics may also suppress LH secretion and result in reversible HH (45). A common side-effect of psychotropic medications such as phenothiazines or risperidone is hyperprolactinemia, which inhibits endogenous GnRH release resulting in HH (46).

Critical Illness

Any period of severe chronic (47), or acute illness such as surgery (48), myocardial infarction (49), burn injury (50), and renal disease (51) may result in low testosterone levels (52). Acute injury is accompanied by a prompt and direct suppression of Leydig cell function (53). When severe stress becomes prolonged, hypogonadotropism ensues largely due to attenuation of pulsatile LH release (53). Endogenous dopamine or opiates may be involved in the pathogenesis of HH induced by critical illness (52).

Structural

Structural lesions of the hypothalamus and pituitary can interfere with the normal pattern of GnRH and/or gonadotropin secretion. The majority of patients with HH secondary to such tumors have multiple pituitary hormone deficiencies in addition to that of gonadotropins (54, 55). However, a mass lesion in the pituitary or hypothalamus is more likely to disrupt the secretion of gonadotropins than that of ACTH or TSH. Thus, patients may present with hypogonadism in the absence of adrenal or thyroid hormone deficiency.

In children, craniopharyngioma is the most common tumor resulting in HH, and is often associated with growth retardation, visual field defects and diabetes insipidus. In adults, prolactinomas are the most frequent cause of HH and may do so by either interfering with GnRH secretion, or in the case of macroadenomas, by local destruction and compression of the gonadotropes. Hyperprolactinemia results in altered dopaminergic function, which has been shown to reduce GnRH mRNA levels and decrease serum levels of LH, FSH and testosterone (56, 57). Although men with hyperprolactinemia may develop galactorrhea, it occurs much less frequently than in women, due to lack of prior stimulation by estrogen and progesterone.

Rarer causes of HH include infiltrative disorders of the hypothalamus or pituitary such as hemochromatosis, sarcoidosis, lymphocytic hypophysitis and histiocytosis, in which case the presence of systemic signs and symptoms frequently leads one to the diagnosis (58-59). Cranial irradiation for the treatment of CNS tumors or leukemia may also result in the gradual onset of hypothalamic-pituitary failure (60). The degree of impairment depends upon both the dose and type of radiation employed and typically reflects hypothalamic dysfunction since the hypothalamus is significantly more radiosensitive than the pituitary. In general, the younger the patient the greater the susceptibility to endocrine dysfunction following radiation therapy (60). Sudden and severe hemorrhage into the pituitary can also results in permanent impairment of pituitary function including hypogonadism (61).

PATHOPHYSIOLOGY

The combined use of both frequent blood sampling and genetic studies has contributed to our understanding of the pathophysiology of isolated GnRH deficiency. Due to its confinement within the hypophyseal-portal blood supply, direct sampling of GnRH is not feasible in the human. In addition, measurements of GnRH in the peripheral circulation do not accurately reflect its secretion due to its rapid half-life of 2-4 min (62, 63). Consequently, inferential approaches must be used to study GnRH secretion in the human.

Traditionally, LH has been used as a surrogate marker of GnRH activity (64, 65). More recently, the pulsatile component of free alpha subunit (FAS) secretion has been shown to be tightly correlated with that of LH (66)and to be driven by GnRH based on its eradication by GnRH receptor blockade (67). Given its half-life of 12-15 min, FAS is useful in tracking GnRH secretion at fast pulse frequencies (68).

Genetics of CHH

Considerable genetic heterogeneity has also been found to underlie CHH. Most cases of CHH are sporadic (80%), suggesting that either the frequency of spontaneous mutations in this disorder is high or that the etiology of many cases is not genetic. In the last decade considerable advances have been made in unraveling the genetic basis for CHH and to date, mutations have been identified in approximately 40% of cases (70). Unique genetic mechanisms have been described for both KS and nCHH. However, some probands with KS have family members with hypogonadism but normal olfaction (Fig. 4) and the same mutations have been shown to be associated with both KS and nCHH suggesting that they should no longer be viewed as distinct diagnostic subsets (71).

Fig. 4. A

family with GnRH deficiency and multiple affected members in two generations. The pedigree is compatible with autosomal dominant transmission with incomplete penetrance. Note the presence of GnRH deficiency both with and without anosmia in the same family.

Human GnRH deficiency has traditionally been viewed as a monogenic disorder that could be inherited as an autosomal dominant, autosomal recessive, or X-linked trait. However, the long appreciated phenotypic variability within and across families with single gene defects combined with the recent demonstration that patients with CHH may harbor a mutation in more than one gene (72, 73) suggest a more complex mode of inheritance. Indeed, a systematic study of nearly 400 patients with isolated GnRH deficiency who were screened for 8 known loci revealed 17% of patients harbor multiple mutations consistent with an oligogenic rather than a monogenic mode of inheritance (74).

Genetic Defects of the Gonadotropins

Genetic defects in the genes encoding LH-b subunit and FSH-b are rare and have been summarized in a recent review (115). The initial case report of a male who failed to go through puberty identified a homozygous mutation in amino acid 54 of the LH-b subunit that rendered LH incapable of binding to its receptor (116). The mutant hormone had no biologic activity but had normal immunoreactivity. Testosterone levels were reduced in association with elevated LH and FSH levels. This clinical picture could easily be confused with primary hypogonadism had the patient not been shown to have a normal serum testosterone response to hCG administration. The first reported male with an FSH-b gene mutation (Cys82Arg missense) appeared normally virilized and had normal testosterone levels, but small testes, azoospermia, a high LH and a low FSH level (117). The second reported case of a man with an FSH-b gene mutation (Val61X) had very small testes (1-2 mL) and an unexpected absence of pubertal development; he was found to harbor an additional defect in Leydig cell function as evidenced by high LH and low testosterone levels (118).

Other rare genetic syndromes associated with hypogonadotropic hypogonadism

HH also occurs as one of a constellation of features in a number of rare syndromes including X-linked adrenal hypoplasia congenita (AHC) caused by defects in the DAX1 (NROB1) gene (Xp21) (119, 120), CHARGE syndrome (coloboma, heart defects, atresia of the choanae, retardation of growth and development, genital anomaly, ear abnormalities) resulting from CHD7 (8q12.1) defects (121, 122), leptin deficiency (123, 124), and prohormone convertase 1/3 deficiency (5q15-21) (125).

In addition, combined pituitary hormone deficiency has been linked with rare abnormalities in genes encoding transcription factors necessary for pituitary development. Mutations in the gene, PROP1 (Prophet of Pit-1) appear to be the most common cause of both familial and sporadic congenital combined pituitary hormone deficiency (126). Most patients with inactivating mutations in PROP1 exhibit low gonadotropin levels and fail to enter puberty as well as having additional deficiencies in GH, PRL, and TSH (126). However, there is a report of two sibships with a homozygous mutation, Arg120Cys in PROP1 in which the affected children entered puberty spontaneously and then developed gonadotropin deficiency over time (127). The same sequence of events was observed in several children with mutations causing a complete loss of function raising questions as to whether this should more correctly be viewed as a model of acquired rather than congenital gonadotropin deficiency (128).

Recently the critical role of PROK2, PROKR2 and FGF8 in the developing pituitary has been noted in the genetic overlap between KS and developmental disorders including combined pituitary hormone deficiency, septo-optic dysplasia, and holoprosencephaly (129, 130). Mutations in the gene SOX2 have been identified in patients with HH in association with anopthalmia or microphhalmia (131, 132), while SOX10 and IL17RD are key genes to consider when deafness is present in association with CHH (106, 133).

EVALUATION

Hypogonadism is defined as a defect in one of the two major functions of the testes i.e. production of testosterone and spermatogenesis. The presence of hypogonadism can reflect disorders intrinsic to the testes (primary or hypergonadotropic hypogonadism) or disorders of the pituitary or hypothalamus (secondary or hypogonadotropic hypogonadism). These two entities can be distinguished by measuring serum LH and FSH concentrations (Fig. 5). Primary hypogonadism is characterized by a low serum testosterone level and oligo- or azoospermia in the presence of elevated serum LH and FSH concentrations. In contrast, secondary hypogonadism is diagnosed in the setting of a low testosterone level and sperm count in association with low or inappropriately normal serum LH and FSH concentrations.

Figure 5Algorithm for evaluation of hypogonadism in a male.

It is best to measure testosterone in a morning sample given the normal diurnal rhythm of testosterone secretion. Repeat measurements should be performed if the initial reading is low. In secondary hypogonadism, measuring the serum LH response to a single bolus of exogenous GnRH is not helpful in distinguishing pituitary from hypothalamic disease, because a subnormal response may occur in both settings. Patients with HH due to congenital absence of hypothalamic GnRH secretion may have had no prior exposure to GnRH; in this setting, repeated administration of GnRH is needed to prime the gonadotropes to elicit a gonadotropin response. Characterization of the pulsatile pattern of LH secretion with frequent blood sampling is useful in refining the diagnosis in a research setting, but is not practical for clinical purposes.

A semen analysis should be obtained to assess sperm production and the count, motility, and morphology determined. Based on WHO criteria, the parameters for a normal semen analysis are a sperm density ³ 15 million sperm/mL of ejaculate with total motility ³ 40% and strictly defined normal morphology of ³ 4% (134). Two normal semen analyses are sufficient to indicate that the sperm density and motility are normal.

In the case of secondary hypogonadism, it is critical to assess the rest of the pituitary axis, including a prolactin level to ensure that the defect is isolated to the HPG axis. Transferrin saturation should be measured to exclude hemochromatosis. Patients with HH typically undergo adrenarche at a normal age and should therefore have normal adult male levels of DHEAS. Radiographic evaluation should include a bone age determination, MRI of the pituitary and hypothalamic area and, in adults, a DEXA scan to assess bone mineral density. Renal ultrasound should be considered to assess for unilateral renal agenesis in patients with KS especially those known to have a KAL1 mutation.

The growing genetic heterogeneity of CHH poses a significant challenge to clinicians in the diagnostic work of patients. While in familial cases the mode of inheritance can be used to guide genetic testing, the majority of CHH cases are actually sporadic. However, a careful clinical evaluation can be helpful in prioritizing genetic testing. In a recent analysis of 219 patients with KS the following clinical features were highly associated with specific gene defects: synkinesia (KAL1), dental agenesis (FGF8/FGFR1), digital bony abnormalities (FGF8/FGFR1), and hearing loss (CHD7) (135). Weaker associations were observed between renal agenesis and KAL1, cleft lip/palate and FGF8/FGFR1, CHD7 and HS6ST1, and male reversal and FGF8/FGFR1 and HS6ST1 (135).

TREATMENT

Choosing the most appropriate treatment for men with HH should be based upon an informed discussion between the patient and his physician. Androgen therapy, whether by exogenous testosterone replacement or induction of endogenous testosterone production by hCG is needed in all HH patients. Androgens play a number of important physiologic roles in the human and are required not only for virilization and normal sexual function, but also for maintenance of both muscle and bone mass, as well as normal mood and cognition.

While testosterone is the primary treatment modality used to induce and maintain secondary sexual characteristics and sexual function in men with hypogonadism, treatment with testosterone does not restore fertility. Therefore, in patients in whom fertility is the treatment goal, induction of gonadotropin secretion by pulsatile GnRH or exogenous gonadotropins is necessary. While hormone therapy is the mainstay of treatment for CHH, cryptorchidism, if present, should be treated surgically with orchidopexy, ideally before the age of 2 years to improve fertility outcomes and reduce the risk of future testicular malignancy (136).

The traditional teaching has been that patients with CHH require life long therapy but the recent demonstration of reversal of hypogonadism suggests that this many not always be the case (113, 137, 138). In a study of a large cohort of men with CHH, 10% of cases demonstrated a sustained reversal after discontinuation of treatment. Reversal occurred across a spectrum of phenotypes including patients with and without anosmia and those with absent or partial spontaneous puberty (137). A key, but sometimes subtle, clinical finding that can predict reversal is spontaneous testicular enlargement on testosterone therapy (137, 138). Reversal has been documented in patients with a variety of genetic defects including GNRHR, FGFR1 and CHD (137, 138) but has been shown to be particularly common in those with defects in the neurokinin pathway (113). While the mechanism of reversal in CHH is not understood it is possible is that exposure to sex steroids may enhance the plasticity of the neuronal network responsible for GnRH secretion. In light of these studies, it seems reasonable to recommend that a brief discontinuation of hormonal therapy be considered in patients with CHH to assess for reversibility. The trial period off treatment will depend on the formulation used but should be approximately 2-3 months for intramuscular testosterone and 4 weeks for those receiving transdermal preparations.

2. Pulsatile GnRH Therapy

The alternative to gonadotropin therapy is pulsatile administration of GnRH, which may be administered by a programmable, portable mini-infusion pump (Fig. 6) (156). While intravenous administration produces the most physiologic GnRH pulse contour and ensuing LH response (157), the subcutaneous route is clearly more practical for the long term treatment required to stimulate spermatogenesis. Based on our normative data (7), the frequency of GnRH administration that we employ is every 2 hours. The dose of GnRH is titrated for each individual to ensure normalization of testosterone, LH and FSH and varies from 25 to 600 ng/kg per bolus. Patients on longterm therapy are monitored with serum testosterone and gonadotropin levels at monthly intervals. Once testicular volume reaches 6-8 mL, regular semen analyses are obtained. The majority of patients require treatment for 18-24 months to maximize testicular growth and achieve spermatogenesis, although the time taken to reach these endpoints tends to be shorter in those with a larger initial gonadal size. In our analysis of 76 men with CHH, pulsatile GnRH restored normal testosterone levels in 93% of cases and was successful in inducing spermatogenesis in 77% by 12 months and 82% by 24 months (158). As with gonadotropin therapy, testicular size is an important predictor of successful treatment while negative predictors include a history of cryptorchidism and a pre-treatment inhibin B level <60 pg/mL (158).

Fig. 6

LH and testosterone (T) at baseline and in response to GnRH therapy in a man with complete GnRH deficiency. Baseline LH secretion pattern is apulsatile with a hypogonadal T level. Exogenous pulsatile GnRH therapy administered every 2 h stimulates normal LH pulses which in turn induce a normal serum T level.

If pulsatile GnRH treatment fails, a mutation of GNRHR should be considered (94) and genetic testing arranged, if available. A second cause of failure of pulsatile GnRH treatment is the appearance of anti-GnRH antibodies, which typically occur in the setting of erratic compliance and are associated with a progressive decrease in T and gonadotropins levels.

Both exogenous gonadotropins and pulsatile GnRH are very effective in stimulating spermatogenesis. Most studies have no shown no advantage of either therapy in terms of testicular growth, onset of spermatogenesis, final sperm counts or pregnancy rates (151, 159).

However, pulsatile GnRH therapy is not approved for induction of spermatogenesis by the Food and Drug Administration in the United States, and its use is thus confined to specialist centers.

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