What is the initial laboratory workup for suspected hypogonadism (low testosterone)?

Laboratory Workup for Suspected Hypogonadism (Low Testosterone)

Initial Diagnostic Testing

The diagnosis of testosterone deficiency requires two separate early morning (8-10 AM) fasting total testosterone measurements, both showing levels below 300 ng/dL, combined with clinical symptoms and/or signs of hypogonadism. 1

Primary Laboratory Tests

• Total testosterone: Measure on two separate mornings between 8-10 AM using the same laboratory and methodology 1, 2

  • Use a total testosterone level below 300 ng/dL as the diagnostic threshold 1
  • Samples must be fasting to ensure accuracy 2
  • Ideally measured by liquid chromatography-tandem mass spectrometry in a CDC-certified laboratory 3

• Free testosterone: Should be measured when total testosterone is near the lower limit of normal (280-400 ng/dL range) or when conditions alter sex hormone-binding globulin 1, 2

  • Free testosterone is a better index of gonadal status than total testosterone 4
  • Must be measured by equilibrium dialysis method for accuracy 1, 2
  • Particularly important in obese patients where total testosterone may be misleadingly low due to reduced SHBG 1, 4

• Sex hormone-binding globulin (SHBG): Measure to calculate free testosterone index (total testosterone/SHBG ratio) 4

  • A ratio <0.3 indicates hypogonadism 4
  • Essential in obese men who may have low total testosterone solely due to low SHBG but normal free testosterone 1

Adjunctive Testing to Determine Etiology

• Luteinizing hormone (LH): Must be measured in all patients with confirmed low testosterone 1, 4

  • Elevated LH indicates primary (testicular) hypogonadism 1
  • Low or inappropriately normal LH indicates secondary (hypothalamic-pituitary) hypogonadism 1

• Follicle-stimulating hormone (FSH): Helps distinguish primary from secondary hypogonadism 1, 4

  • Elevated in primary hypogonadism 1
  • Low or normal in secondary hypogonadism 1

• Prolactin: Check in patients with low testosterone combined with low or inappropriately normal LH levels 4

  • Elevated prolactin suggests prolactinoma as the cause of secondary hypogonadism 1

Additional Testing in Specific Scenarios

• Iron saturation: Measure if secondary hypogonadism is present to rule out hemochromatosis 1

• Thyroid function tests: Perform to exclude thyroid disorders that may affect testosterone levels 4

• Bone density and metabolic markers: Consider measuring corrected serum calcium, serum phosphate, and 25-OH vitamin D, especially if osteoporosis is suspected 4

• Pituitary imaging (MRI of sella turcica): Indicated if secondary hypogonadism is confirmed with low LH/FSH to identify structural pituitary or hypothalamic lesions 1

Clinical Correlation Requirements

Laboratory diagnosis alone is insufficient—symptoms and/or signs must be present to diagnose testosterone deficiency. 1, 2

Key Symptoms to Assess:

• Reduced energy, endurance, and physical performance 1
• Fatigue and reduced motivation 1
• Depression, poor concentration, and impaired memory 1
• Reduced libido and erectile dysfunction 1
• Infertility 1
• Visual field changes (bitemporal hemianopsia) or anosmia suggesting pituitary pathology 1

Key Physical Examination Signs:

• Body habitus and virilization status (body hair patterns in androgen-dependent areas) 1
• Body mass index or waist circumference 1
• Gynecomastia 1
• Testicular size, consistency, and presence of masses 1
• Varicocele presence 1
• Prostate size and morphology 1

High-Risk Populations Requiring Screening

Measure testosterone even without symptoms in patients with: 1, 4

• Unexplained anemia 1
• Bone density loss 1
• Diabetes 1
• Exposure to chemotherapy or testicular radiation 1
• HIV/AIDS 1
• Chronic narcotic use 1
• Male infertility 1
• Pituitary dysfunction 1
• Chronic corticosteroid use 1

Critical Pitfalls to Avoid

• Do not diagnose hypogonadism based on a single testosterone measurement 1, 2

  • Natural variability requires confirmation with a second morning sample 1

• Do not rely on total testosterone alone when levels are 280-400 ng/dL 5

  • Total testosterone between 280-350 ng/dL is not sensitive enough to exclude hypogonadism 5
  • Free testosterone measurement is essential in this range 2, 5

• Do not fail to distinguish primary from secondary hypogonadism 4

  • These require different management approaches and further workup 1

• Do not use screening questionnaires as a substitute for laboratory testing 1

  • Questionnaires have variable sensitivity and specificity and should not replace proper evaluation 1

• Do not obtain samples at non-morning times 4

  • Testosterone levels are highest in the morning and decline throughout the day 1, 2

• Do not treat based on symptoms alone without laboratory confirmation 1, 2

  • Both low testosterone levels AND symptoms are required for diagnosis 1