What is the diagnosis for a 55‑year‑old male with consistently low total and free testosterone, normal SHBG, low‑normal FSH and LH, and normal prolactin levels?

Secondary (Hypogonadotropic) Hypogonadism

This 55-year-old male has secondary (hypogonadotropic) hypogonadism, characterized by consistently low testosterone with inappropriately low-normal gonadotropins (FSH and LH), indicating hypothalamic-pituitary axis dysfunction rather than primary testicular failure 1.

Diagnostic Confirmation

The laboratory pattern is diagnostic:

• Total testosterone: 5.5-7.2 nmol/L (consistently below 10 nmol/L reference range)
• Free testosterone: 128-163 pmol/L (below normal range of 225-725 pmol/L)
• FSH and LH: 2 and 3 IU/L respectively—these are inappropriately low-normal for the degree of testosterone deficiency
• Normal SHBG: 21-23 nmol/L rules out SHBG-related alterations
• Normal prolactin: 150 mIU/L excludes hyperprolactinemia

In primary hypogonadism, FSH and LH would be markedly elevated (>9 and >8 IU/L respectively) as the pituitary attempts to compensate for testicular failure. The low-normal gonadotropins despite profound testosterone deficiency confirm hypothalamic-pituitary dysfunction 1, 2.

Classification: Functional vs. Organic

The most likely diagnosis is functional secondary hypogonadism rather than organic (structural) disease, given the clinical context 1. The 2025 EAU guidelines emphasize this critical distinction because it determines both reversibility and treatment approach.

Investigate These Specific Causes of Functional Hypogonadism:

Metabolic conditions (most common in this age group):

• Obesity/metabolic syndrome: Check BMI and waist circumference—obesity is the leading reversible cause 1, 3
• Type 2 diabetes: Strongly associated with secondary hypogonadism 1
• Insulin resistance: Even without overt diabetes 1

Medications that suppress the HPG axis 1:

• Opioids (chronic use)
• Glucocorticoids
• Exogenous testosterone or anabolic steroids (past or current use)
• Medications causing hyperprolactinemia

Other systemic conditions:

• Chronic illness
• Sleep apnea
• Nutritional deficiencies

Rule Out Organic Causes:

The lab report suggests checking additional tests if not already done:

• Iron studies: Hemochromatosis causes secondary hypogonadism
• TSH with free T4: Hypothyroidism can suppress gonadotropins 1
• Morning cortisol: Cushing's disease or adrenal insufficiency 1
• MRI pituitary: Only if clinical suspicion for pituitary/hypothalamic mass, especially if visual field defects, headaches, or other pituitary hormone deficiencies 1, 2

Critical Management Algorithm

Step 1: Address Reversible Causes FIRST

Weight loss is the first-line treatment if obesity is present 1. The 2025 EAU guidelines are explicit: functional hypogonadism from obesity and metabolic disorders should be treated by resolving underlying conditions before considering testosterone therapy. Meta-analyses confirm that weight loss through low-calorie diets reverses obesity-associated secondary hypogonadism by improving testosterone levels and normalizing gonadotropins 1.

Discontinue or modify interfering medications if possible 1.

Treat underlying conditions:

• Optimize diabetes control
• Treat sleep apnea if present
• Address thyroid dysfunction

Step 2: Testosterone Therapy Considerations

Testosterone therapy should only be initiated if:

1. Symptoms are present and confirmed (decreased libido, erectile dysfunction, reduced energy, etc.) 1, 3
2. Reversible causes have been addressed or cannot be modified 1
3. The patient is not seeking fertility (testosterone suppresses spermatogenesis) 1, 2

Important distinction: Unlike primary hypogonadism where testosterone is the only option, secondary hypogonadism patients can potentially achieve both fertility preservation AND normal testosterone with gonadotropin therapy if fertility is desired 1, 2.

Step 3: If Testosterone Therapy Is Initiated

Monitor for:

• Erythrocytosis (hematocrit >54%)—the most significant adverse effect with 8-fold increased risk 2
• Serum testosterone levels to ensure therapeutic range
• Symptom improvement (libido, erectile function, energy)
• PSA in men >40 years

Common Pitfalls to Avoid

1. Do not start testosterone without investigating the cause: Secondary hypogonadism requires evaluation for pituitary pathology and reversible causes 1, 4

2. Do not ignore the fertility question: Testosterone therapy will suppress spermatogenesis; if fertility is desired now or in the future, gonadotropin therapy is the appropriate choice 1, 2

3. Do not skip lifestyle modification: The evidence shows combining lifestyle changes with testosterone yields better outcomes than either alone, but lifestyle modification should be attempted first in functional hypogonadism 1

4. Do not measure testosterone during acute illness: This will give falsely low results 1

5. Do not rely on total testosterone alone in obese patients: Obesity lowers SHBG, but this patient's SHBG is normal, so calculated free testosterone accurately reflects androgen status 3, 5

The diagnosis is clear-cut secondary hypogonadism; the critical next step is determining whether this is functional (reversible) or organic (permanent), which fundamentally changes the treatment approach and prognosis 1.