What laboratory tests are recommended for a male with low libido?

Laboratory Testing for Male with Low Libido

In a male presenting with low libido, measure a morning (8-10 AM) total testosterone level using an accurate assay, and if levels are low or borderline, add free testosterone (preferably by equilibrium dialysis), sex hormone-binding globulin (SHBG), and serum luteinizing hormone (LH) to guide further evaluation. 1

Initial Hormonal Assessment

Morning Total Testosterone

• Draw blood between 8 AM and 10 AM for total testosterone measurement 1
• Low testosterone is generally defined as <300 ng/dL (though laboratory ranges vary from 300-800 ng/dL) 1
• If abnormal, repeat the test on a separate occasion to confirm the diagnosis before proceeding 1

Free Testosterone and SHBG

• Measure free testosterone by equilibrium dialysis (the gold standard method) when total testosterone is low or borderline 1
• This is particularly critical in men with obesity, as they often have low SHBG, which can cause low total testosterone despite normal free testosterone 1
• Calculate or measure SHBG and albumin to distinguish true hypogonadism from SHBG-related changes 1
• Men over 60 years have elevated SHBG prevalence and may have normal total testosterone but low free testosterone in up to 26% of cases 2

Secondary Hormonal Testing (When Testosterone is Confirmed Low)

Luteinizing Hormone (LH)

• Measure serum LH in all patients with confirmed low testosterone 1
• Low or inappropriately normal LH with low testosterone indicates secondary (central) hypogonadism from hypothalamic-pituitary dysfunction 1
• Elevated LH with low testosterone indicates primary (testicular) hypogonadism 1

Prolactin

• Measure serum prolactin only in men with low testosterone AND low/normal LH 1
• If prolactin is elevated, repeat the measurement to exclude spurious results 1
• Persistently elevated prolactin requires referral to endocrinology and evaluation for prolactinoma 1
• Men with total testosterone <150 ng/dL and low/normal LH should undergo pituitary MRI regardless of prolactin levels, as non-secreting adenomas may be present 1

Follicle-Stimulating Hormone (FSH)

• Measure FSH in men interested in fertility preservation 1
• Elevated FSH with low testosterone suggests impaired spermatogenesis and warrants consideration of semen analysis 1

Additional Considerations

When Prolactin Testing is NOT Needed

• Prolactin measurement is unnecessary in men with normal testosterone or those with elevated LH (primary hypogonadism) 3
• A history of decreased libido alone cannot predict which patients will have hormonal abnormalities 3

Estradiol Testing

• Measure serum estradiol only if the patient presents with breast symptoms or gynecomastia prior to testosterone therapy 1
• Elevated baseline estradiol requires endocrinology referral 1

Common Pitfalls to Avoid

• Do not rely solely on total testosterone in men over 60 years or those with obesity, as SHBG alterations can mask true hypogonadism 1, 2
• Do not skip the repeat testosterone measurement—single abnormal values require confirmation 1
• Do not measure prolactin in all men with low libido—it is only indicated when testosterone is low with concurrent low/normal LH 1, 3
• Screening questionnaires are not appropriate substitutes for laboratory testosterone measurement 1

High-Risk Populations Requiring Testing Even Without Symptoms

Consider testosterone testing even in asymptomatic men with: unexplained anemia, bone density loss, diabetes, obesity, HIV, chronic narcotic use, history of chemotherapy or testicular radiation, infertility, pituitary disorders, or chronic corticosteroid use 1