What lab tests are recommended for evaluating low libido in men?

Laboratory Testing for Low Libido in Men

For men with low libido, the recommended laboratory evaluation should include morning total testosterone, free or bioavailable testosterone, luteinizing hormone (LH), follicle-stimulating hormone (FSH), prolactin, estradiol, hemoglobin A1c or fasting glucose, and a lipid panel. 1

Primary Laboratory Tests

Hormonal Evaluation

• Total testosterone: Morning measurements between 8-10 AM are essential for accuracy
  • Repeat testing is necessary to confirm abnormal results 1
• Free or bioavailable testosterone: Critical when total testosterone is borderline low or when SHBG abnormalities are suspected
  • 17.2% of men with sexual dysfunction have normal total testosterone but low free testosterone 2
  • Measurement of free testosterone can reduce unnecessary endocrine evaluations by 50% 2
• Luteinizing hormone (LH) and follicle-stimulating hormone (FSH): Essential to distinguish between primary and secondary hypogonadism 1
• Prolactin: Should be measured even if testosterone is normal, as hyperprolactinemia can cause low libido independently 1
• Estradiol: Particularly important if breast symptoms or gynecomastia are present 1

Metabolic Evaluation

• Hemoglobin A1c or fasting glucose: Diabetes can significantly affect testosterone levels and sexual function 1
• Lipid panel: Important for cardiovascular risk assessment, which is linked to sexual function 1

Additional Considerations

When to Consider Specialized Testing

• Sex hormone-binding globulin (SHBG): When total testosterone is normal but symptoms persist
  • Evaluate for underlying causes of elevated SHBG (liver disease, HIV, hyperthyroidism, aging) 1
• Thyroid function tests: Consider when symptoms suggest thyroid dysfunction
• Complete blood count: To rule out anemia or other hematologic conditions

Important Clinical Pearls

1. A history of decreased libido or testicular atrophy on physical examination cannot reliably predict hypogonadism, making laboratory testing essential 2
2. Compensated hypogonadism (normal/high total testosterone with elevated LH) may present with hypogonadal symptoms despite normal total testosterone 1
3. Low libido can exist with or without other sexual dysfunctions; it's comorbid with erectile dysfunction in 38% of cases 3
4. Primary reduced libido (not associated with hypogonadism, hyperprolactinemia, or psychopathology) has different characteristics than secondary reduced libido 3

Interpretation and Follow-up

• If testosterone is low, repeat testing is necessary to confirm the diagnosis
• If prolactin is elevated, further evaluation for pituitary adenoma may be warranted
• If testosterone is low and LH/FSH are elevated, this indicates primary hypogonadism
• If testosterone is low and LH/FSH are low or normal, this suggests secondary hypogonadism

Remember that while laboratory testing is essential, a comprehensive evaluation should also consider psychological factors, relationship issues, and medication effects that may contribute to low libido but won't be detected through laboratory testing alone.