What lab tests are recommended for screening for low sexual dysfunction, specifically hypogonadism (low testosterone levels)?

Laboratory Screening for Sexual Dysfunction and Hypogonadism

For men with symptoms of sexual dysfunction, particularly erectile dysfunction or decreased libido, a morning serum testosterone level should be measured as the primary screening test for hypogonadism. 1

Initial Laboratory Evaluation

Primary Screening Tests:

• Total testosterone: Morning sample (8-10 AM) when testosterone levels are highest 1
• Sex hormone-binding globulin (SHBG): Important for calculating free testosterone, especially in men with total testosterone levels near the lower limit 1
• Free or bioavailable testosterone: Should be measured in men with total testosterone levels close to the lower limit, as diabetes and obesity can affect SHBG levels 1

Secondary Tests (Based on Initial Results):

• Luteinizing hormone (LH) and follicle-stimulating hormone (FSH): To distinguish between primary and secondary hypogonadism 1
• Prolactin: Elevated levels can indicate pituitary tumors and cause sexual dysfunction despite normal testosterone 2
• Estradiol: Should be measured in men presenting with breast symptoms or gynecomastia 1

Special Considerations

For Men with Total Testosterone <150 ng/dL:

• Pituitary MRI: Required if combined with low or low/normal LH, regardless of prolactin levels, to rule out non-secreting adenomas 1

For Men with Persistently Elevated Prolactin:

• Referral to endocrinology: For evaluation of potential pituitary tumors such as prolactinomas 1

Additional Recommended Tests

Cardiovascular Risk Assessment:

• Lipid panel: As erectile dysfunction is often an early marker for cardiovascular disease 1, 2
• Blood pressure measurement 1
• Fasting glucose or HbA1c: To screen for diabetes 1
• Electrocardiogram (ECG): Particularly in men with hypertension or diabetes 1

Other Relevant Tests:

• Hemoglobin/hematocrit: Required prior to initiating testosterone therapy 1
• PSA: For men over 40 years before starting testosterone therapy 1

Interpretation of Results

• Low testosterone: <300 ng/dL is generally considered low, but symptoms may occur at higher levels 1
• Normal total testosterone with low free testosterone: May occur with elevated SHBG, requiring calculation of free testosterone 2
• Low testosterone with elevated LH/FSH: Indicates primary hypogonadism (testicular failure) 3
• Low testosterone with low/normal LH/FSH: Indicates secondary hypogonadism (hypothalamic-pituitary dysfunction) 3

Clinical Pearls

• Morning testosterone measurement is essential as levels fluctuate throughout the day 1
• Repeat abnormal testosterone measurements to confirm diagnosis 1
• Obesity is a major confounder in testosterone measurement 1
• Men with diabetes have lower mean testosterone levels compared to age-matched men without diabetes 1
• Erectile dysfunction often precedes cardiovascular events and should prompt cardiovascular risk assessment 1
• Consider medication review, as many drugs can contribute to sexual dysfunction 1

By following this laboratory screening approach, clinicians can effectively identify and characterize hypogonadism in men presenting with sexual dysfunction, allowing for appropriate treatment decisions based on the underlying etiology.