Which laboratory tests should be obtained to evaluate low testosterone in a male patient?

Laboratory Evaluation for Low Testosterone

Measure fasting morning total testosterone on two separate occasions using a validated assay; levels below 300 ng/dL combined with consistent symptoms confirm the diagnosis of testosterone deficiency. 1, 2, 3

Initial Testing Protocol

Total Testosterone Measurement

• Obtain blood samples before 10 AM when testosterone levels peak due to circadian variation 2, 3, 4
• Repeat the morning total testosterone measurement on a second occasion to confirm persistently low levels, as single measurements are unreliable due to intra-individual daily variability 2, 5, 6
• Use a well-validated assay with demonstrated accuracy; many commercial immunoassays lack precision at lower testosterone ranges and mass spectrometry is considered the gold standard 7, 4, 8
• Ensure the patient is fasting, as metabolic factors can influence results 3
• Avoid testing during acute illness, as stress and inflammatory states transiently suppress testosterone production 1

Diagnostic Threshold

• Total testosterone below 300 ng/dL is the accepted cut-off to support testosterone deficiency diagnosis in symptomatic men 2, 5, 6

Follow-Up Laboratory Tests

Free or Bioavailable Testosterone

• Measure free testosterone by equilibrium dialysis (gold standard) or calculate it using a validated formula when total testosterone is borderline (near 300 ng/dL) or when conditions affecting sex hormone-binding globulin (SHBG) are present 9, 5, 6
• Check SHBG levels to understand testosterone binding dynamics, particularly in obese men who typically have low SHBG 9
• Avoid direct free testosterone immunoassays, which are unreliable; use calculated surrogates instead 4, 6

Determining Etiology: Primary vs. Secondary Hypogonadism

• Measure serum luteinizing hormone (LH) and follicle-stimulating hormone (FSH) to distinguish primary hypogonadism (elevated gonadotropins) from secondary hypogonadism (low or inappropriately normal gonadotropins) 9, 2, 3
• This distinction is critical because secondary hypogonadism patients may achieve fertility with gonadotropin therapy, whereas primary hypogonadism patients require testosterone replacement that suppresses spermatogenesis 1

Prolactin Assessment

• Check serum prolactin levels in men with low testosterone combined with low or low-normal LH, as hyperprolactinemia is a reversible cause of secondary hypogonadism 2, 5, 6

Thyroid Function

• Measure thyroid-stimulating hormone (TSH) and free T4, as thyroid disorders affect sex hormone metabolism and can contribute to hypogonadal symptoms 9

Metabolic Screening

• Obtain fasting glucose or HbA1c to screen for diabetes, which is strongly associated with testosterone deficiency and sexual dysfunction 3
• Check a lipid profile to assess cardiovascular risk, as low testosterone and erectile dysfunction are independent markers for future cardiac events 3

Additional Considerations

Adrenal Function

• DHEA and progesterone levels are not routinely necessary unless specific adrenal pathology is suspected 9

Avoid Common Pitfalls

• Do not rely on a single testosterone measurement; at least two morning samples are required for diagnosis 2, 5, 6
• Do not use non-morning samples, which may be falsely low due to diurnal variation 2, 4
• Do not use screening questionnaires alone without laboratory confirmation, as they lack specificity 1, 2
• Do not order testosterone testing during acute illness, as results will be transiently suppressed and misleading 1

When to Measure Additional Hormones

• In men with borderline testosterone (approximately 231-346 ng/dL), more detailed hormonal evaluation including free testosterone and gonadotropins helps clarify whether treatment is warranted 3
• In obese men, recognize that increased aromatization of testosterone to estradiol in adipose tissue causes estradiol-mediated negative feedback that suppresses LH secretion, creating functional hypogonadism 9