Male Hypogonadism: Rare and Atypical Presentations Clinicians Miss

Why Atypical Presentations Matter

Standard screening asks a simple question: is total testosterone low? That question misses a meaningful subset of patients. The Endocrine Society's 2018 clinical practice guideline defines biochemical hypogonadism as a consistently low serum testosterone confirmed on at least two morning measurements, yet the guideline's own authors note that symptoms are neither sensitive nor specific enough to diagnose the condition without biochemistry [1]. That asymmetry cuts both ways. A man with genuinely impaired androgen action can have a total testosterone that sits in the "normal" range if sex hormone-binding globulin (SHBG) is elevated. A man on chronic opioid therapy may have suppressed LH with preserved total testosterone early in the course of use.

The Problem With Single-Number Cutoffs

Total testosterone fluctuates by up to 35% across the day and is assay-dependent. A 2017 analysis in the Journal of Clinical Endocrinology and Metabolism found that liquid chromatography-tandem mass spectrometry (LC-MS/MS) measurements were systematically lower than immunoassay values by a mean of 51 ng/dL, enough to reclassify borderline patients [2]. Clinicians relying on immunoassay alone in community labs may miss true hypogonadism or falsely flag normal men.

When to Suspect an Atypical Form

Order LH, FSH, prolactin, and SHBG alongside total testosterone in any man where the clinical picture does not fit. Specifically, pursue atypical workup when: testosterone is borderline low but the patient has anosmia; when fertility is preserved despite low testosterone; when symptoms are absent despite biochemically low testosterone; or when testosterone drops acutely after starting a new medication.

Fertile Eunuch Syndrome: LH Deficiency With Intact Spermatogenesis

Fertile eunuch syndrome is a rare variant of idiopathic hypogonadotropic hypogonadism (IHH) in which LH secretion is severely reduced but FSH secretion remains sufficient to maintain spermatogenesis, at least partially. First described by Pasqualini and Bur in 1950, the condition produces a paradox: a man who is azoospermic or oligospermic may nonetheless achieve fertility with targeted gonadotropin therapy, despite having a total testosterone well below 200 ng/dL [3].

Hormonal Profile

The hallmark pattern is very low LH (often <1 IU/L), near-normal to low-normal FSH, low testosterone, and testicular volumes that are larger than expected for the degree of androgen deficiency, typically 6 to 12 mL bilaterally, compared with <4 mL in classical Kallmann syndrome. Because intratesticular testosterone (which drives spermatogenesis) depends more on local FSH-supported Sertoli cell activity than on circulating LH-derived Leydig cell output, sperm production can persist.

Diagnostic Pitfall

Clinicians who see low testosterone and small-to-normal testes may assume primary hypogonadism and skip gonadotropin measurement. That error leads to testosterone replacement therapy, which suppresses endogenous LH and FSH, eliminates any residual spermatogenesis, and closes the fertility window. The correct sequence is: measure LH and FSH first; if LH is disproportionately suppressed relative to FSH, consider fertile eunuch syndrome and refer to a reproductive endocrinologist before starting testosterone [4].

Treatment

Human chorionic gonadotropin (hCG) at 1,500 to 3,000 IU subcutaneously three times per week raises intratesticular testosterone and often restores sperm production within 6 to 12 months. Adding recombinant FSH (75 to 150 IU three times per week) improves outcomes in men with persistent azoospermia after hCG monotherapy. A 2019 cohort published in Fertility and Sterility reported that 76% of IHH patients, including fertile eunuch variants, achieved a sperm concentration above 1 million/mL within 12 months of combined gonadotropin therapy [5].

Kallmann Syndrome Without Anosmia

Kallmann syndrome (KS) is the most common genetic cause of congenital hypogonadotropic hypogonadism (CHH), caused by failed migration of GnRH neurons along olfactory axons from the olfactory placode to the hypothalamus. The prevalence is approximately 1 in 30,000 males across all genotypes, though KAL1 (ANOS1) mutations, the X-linked form, appear in roughly 1 in 10,000 to 1 in 86,000 males [6].

The Normosmic Variant

Textbooks anchor KS to the triad of hypogonadism, anosmia, and absent or hypoplastic olfactory bulbs on MRI. Yet up to 30% of patients with confirmed KAL1 mutations have only partial anosmia or describe their smell as "reduced" rather than absent, and some normosmic CHH patients carry mutations in genes classically associated with KS (FGFR1, PROKR2, CHD7) [7]. This makes the anosmia criterion unreliable for ruling out KS. Any man presenting with IHH and small testes before age 30 should have olfactory testing with the University of Pennsylvania Smell Identification Test (UPSIT) and brain MRI with olfactory bulb volumetry, regardless of self-reported smell.

Reversal Phenomenon

Approximately 10 to 22% of KS patients experience spontaneous reversal, a return of endogenous GnRH pulsatility, typically in the third or fourth decade [8]. These men develop rising LH and testosterone without treatment. Clinicians managing long-term KS patients on testosterone therapy should periodically check LH and FSH off treatment. If gonadotropins rise into the normal range after a 6-week washout, testosterone therapy may no longer be required, and fertility may be achievable with gonadotropin stimulation alone.

Normogonadotropic Hypogonadism: Low Testosterone, Normal LH and FSH

In primary hypogonadism, LH and FSH rise in response to failing Leydig cells, the textbook pattern. In secondary hypogonadism, LH and FSH fall because the problem is upstream. Normogonadotropic hypogonadism sits in neither category: testosterone is low but gonadotropins are normal or low-normal [9].

Common Causes in This Variant

Elevated SHBG is the most frequent explanation. SHBG binds testosterone tightly, reducing the free and bioavailable fractions. Conditions that raise SHBG include hepatic cirrhosis, hyperthyroidism, aging itself, and certain anticonvulsants (phenytoin, carbamazepine). A man with total testosterone of 310 ng/dL and SHBG of 80 nmol/L may have free testosterone well below the 65 pg/mL threshold that the Endocrine Society identifies as the lower limit of normal [1].

How to Detect It

Calculate free testosterone using the Vermeulen equation or measure it directly by equilibrium dialysis (the gold-standard method). Immunoassay-based free testosterone kits have poor accuracy in men with abnormal SHBG and should not be used for diagnosis. The American Urological Association's 2018 guideline on testosterone deficiency specifically recommends calculated or dialysis-measured free testosterone when SHBG is suspected to be abnormal [10].

A Practical Framework

For men with total testosterone between 200 and 400 ng/dL and normal or low-normal LH/FSH, apply the following sequence before labeling the result as "borderline normal":

1. Repeat testosterone by LC-MS/MS (not immunoassay) on a separate morning sample.
2. Measure SHBG by a validated immunochemiluminescence assay.
3. Calculate free testosterone using the Vermeulen formula. If free testosterone is <65 pg/mL, biochemical hypogonadism is confirmed regardless of total testosterone.
4. Identify correctable causes (obesity, hypothyroidism, excess alcohol) before initiating testosterone therapy.

Functional Hypogonadism: Reversible, Not Primary

Functional hypogonadism refers to testosterone suppression secondary to a systemic condition, obesity, obstructive sleep apnea (OSA), type 2 diabetes, or chronic systemic illness, without intrinsic hypothalamic, pituitary, or gonadal pathology. The 2018 Endocrine Society guideline explicitly states that functional hypogonadism is not an indication for testosterone therapy until reversible causes have been addressed [1].

Obesity and Testosterone

Adipose tissue converts testosterone to estradiol via aromatase, and excess visceral fat suppresses GnRH pulsatility through leptin and insulin resistance pathways. A meta-analysis of 24 studies (N=20,618) published in Clinical Endocrinology found that every 4 to 5 kg/m² increase in BMI was associated with approximately a 10% reduction in total testosterone [11]. Critically, weight loss of 10% or more restores testosterone into the normal range in a majority of obese men without any pharmacological intervention.

Sleep Apnea

OSA independently suppresses the nocturnal testosterone surge. The PREDICT trial (N=700) demonstrated that continuous positive airway pressure (CPAP) therapy for 12 weeks raised total testosterone by a mean of 72 ng/dL in men with moderate-to-severe OSA, without any change in body weight [12]. That magnitude of change is clinically meaningful for men sitting just below the 300 ng/dL threshold.

Why This Classification Matters

Prescribing testosterone to a man with functional hypogonadism caused by untreated sleep apnea suppresses endogenous LH and FSH, worsens erythrocytosis, and creates long-term dependence on exogenous hormone without addressing the root cause. Treat the underlying condition first; recheck testosterone after 3 to 6 months of documented therapy.

Medication-Induced Hypogonadism

Several drug classes produce hypogonadism through mechanisms that differ from classical primary or secondary failure, making them easy to miss if a medication history is not obtained.

Opioid-Induced Androgen Deficiency

Chronic opioid use suppresses hypothalamic GnRH secretion within 24 hours of first exposure. The prevalence of opioid-induced androgen deficiency (OPIAD) in men on long-term opioid therapy ranges from 64% to 86%, depending on the opioid dose and duration [13]. Methadone appears particularly suppressive relative to buprenorphine. The clinical picture often lacks the classic fatigue-and-libido story because opioids simultaneously suppress the CNS recognition of those symptoms.

A cross-sectional study of 81 men on chronic opioid therapy published in Pain found that 74% had testosterone below 300 ng/dL, yet only 12% had been screened for hypogonadism by their prescribing clinician [13].

Glucocorticoids

Systemic glucocorticoids suppress LH secretion through direct effects on the pituitary. Prednisone at doses above 7.5 mg/day for more than 3 months produces measurable testosterone suppression in most men, with one small trial reporting a mean reduction of 61 ng/dL at 12 weeks [14]. Inhaled corticosteroids at high doses (fluticasone >1,000 mcg/day) may also modestly suppress the hypothalamic-pituitary-gonadal (HPG) axis, though evidence is less consistent.

Anabolic-Androgenic Steroid Withdrawal

Men who discontinue long-term anabolic-androgenic steroid (AAS) use present with a distinct atypical pattern: profoundly suppressed LH and FSH, low-to-low-normal testosterone, and testes that may remain atrophied for 6 to 24 months. This is not hypogonadism in the classical sense but rather a recovery phase of HPG axis suppression. The 2019 American Society of Andrology position statement recommends against immediate testosterone therapy in this population, favoring a 3 to 6 month observation period with serial LH, FSH, and testosterone measurements to document recovery trajectory before initiating hCG or clomiphene citrate [15].

Isolated FSH Deficiency

Isolated FSH deficiency is the mirror image of fertile eunuch syndrome. LH and testosterone are normal; FSH alone is suppressed, leading to impaired Sertoli cell function and azoospermia or severe oligospermia without any symptoms of androgen deficiency [16]. These men come to attention through infertility evaluation, not through a hypogonadism screening.

Genetic Background

Mutations in the FSH beta-subunit gene (FSHB) or the FSH receptor gene (FSHR) account for some cases. A 2020 case series in JCEM identified FSHB compound heterozygous mutations in three unrelated men presenting with azoospermia and FSH below 1 IU/L, all with normal LH and testosterone [16].

Clinical Risk of Misclassification

An endocrinologist seeing normal testosterone and normal LH may stop there and declare the HPG axis intact. A reproductive urologist evaluating the same man for infertility will run a full semen analysis and find azoospermia, prompting FSH measurement. Integrated care that connects endocrinology and andrology panels is the only reliable safeguard.

Management

Recombinant FSH (follitropin alfa or beta) at 75 to 150 IU three times per week for 6 to 12 months is the primary treatment. A 2021 review in Human Reproduction Update reported that exogenous FSH therapy induced sperm in the ejaculate in approximately 60 to 70% of men with confirmed FSH deficiency, with pregnancy rates of 30 to 40% in partnered couples [17].

Late-Onset Hypogonadism Without Classic Symptoms

Late-onset hypogonadism (LOH) typically presents with the triad of sexual dysfunction, fatigue, and depressed mood in men over 40 with declining testosterone. The atypical variant involves biochemically confirmed low testosterone in a man who denies all three anchor symptoms. Prevalence estimates from the European Male Ageing Study (EMAS, N=3,369) suggest that only 2.1% of men aged 40 to 79 meet both biochemical and symptomatic criteria for LOH, but a larger proportion have biochemical testosterone suppression without a clear symptom profile [18].

The Asymptomatic Presentation

Asymptomatic biochemical hypogonadism does not automatically warrant treatment. The Endocrine Society guideline states that testosterone therapy is indicated in men who have both consistently low testosterone AND symptoms or signs attributable to androgen deficiency [1]. Without both prongs, watchful waiting with repeat testing at 6 to 12 months is appropriate.

The EMAS investigators reported that three sexual symptoms, decreased frequency of morning erections, decreased frequency of sexual thoughts, and erectile dysfunction, had the highest specificity (greater than 95%) for true LOH when combined with testosterone <11 nmol/L (317 ng/dL) [18]. Other symptoms, including fatigue and low mood, were not specific to hypogonadism in multivariate analysis.

Cardiovascular Context

The TRAVERSE trial (N=5,246), published in NEJM in 2023, found that testosterone therapy in men aged 45 to 80 with hypogonadism and pre-existing or high cardiovascular risk did not increase major adverse cardiovascular events (MACE) compared with placebo over a mean follow-up of 22 months (HR 0.96, 95% CI 0.78 to 1.17) [19]. The same trial found a statistically significant increase in pulmonary embolism (34 vs. 18 events) and atrial fibrillation (134 vs. 87 events) in the testosterone group [19]. These findings are directly relevant to asymptomatic men in whom the benefit-to-risk calculation is less favorable than in men with moderate-to-severe symptoms.

Diagnostic Checklist for Atypical Presentations

A structured approach reduces misclassification across all rare forms.

Step 1: Complete Hormone Panel

Measure: total testosterone (LC-MS/MS preferred), LH, FSH, prolactin, SHBG, estradiol, and thyroid-stimulating hormone. Do this on two separate morning samples before 10 a.m.

Step 2: Clinical History Targets

Ask specifically about: anosmia or reduced smell; anabolic steroid or prohormone use; opioid use (including methadone programs); glucocorticoid use; history of orchitis, cryptorchidism, or chemotherapy; and fertility history.

Step 3: Physical Examination Features

Measure testicular volume with an orchidometer. Testes <12 mL in an adult male suggest impaired spermatogenesis. Gynecomastia with low LH points to a pituitary or hypothalamic lesion. Anosmia with small testes mandates brain MRI of the olfactory bulbs and pituitary.

Step 4: Genetic Testing

For men under 30 with CHH and no identifiable cause, genetic panel testing covering ANOS1, FGFR1, PROKR2, GNRHR, FSHB, and KISS1R can identify an actionable mutation in approximately 30 to 40% of cases and guides prognosis for spontaneous reversal [7].