What laboratory tests are used to diagnose hypogonadism (low testosterone)?

From the Guidelines

To diagnose low testosterone, the primary lab test is total testosterone, which should be measured in the morning (between 8-10 AM) when levels are highest, with a threshold of below 300 ng/dL considered low, as recommended by the American Urological Association guideline 1. The diagnosis of low testosterone requires both a low testosterone measurement and the presence of select symptoms and/or signs.

• The threshold for low testosterone is consistently <300 ng/dL on at least two serum total testosterone measurements obtained in an early morning fashion, preferably using the same laboratory with the same method/instrumentation for measurement 1.
• Clinicians should consider measuring total testosterone in patients with a history of unexplained anemia, bone density loss, diabetes, exposure to chemotherapy, exposure to testicular radiation, HIV/AIDS, chronic narcotic use, male infertility, pituitary dysfunction, and chronic corticosteroid use even in the absence of symptoms or signs associated with testosterone deficiency 1.
• If total testosterone is low, additional tests should include free testosterone (the biologically active form), luteinizing hormone (LH) and follicle-stimulating hormone (FSH) to determine if the cause is primary (testicular) or secondary (pituitary/hypothalamic), as suggested by the Standards of Medical Care in Diabetes-2021 1.
• Other helpful tests include complete blood count, comprehensive metabolic panel, prolactin levels, and thyroid function tests to rule out other conditions that might affect testosterone.
• For accurate results, testosterone should be measured on two separate occasions, preferably 1-2 weeks apart, as levels can fluctuate, and patients should avoid biotin supplements for at least 72 hours before testing as they can interfere with results.
• The most recent guideline from the Standards of Care in Diabetes-2023 also recommends considering screening with a morning serum testosterone level in men with diabetes who have symptoms or signs of hypogonadism, such as decreased sexual desire (libido) or activity, or erectile dysfunction 1.

From the Research

Labs for Low Testosterone

To diagnose low testosterone, several lab tests are used, including:

• Measurement of morning testosterone levels on two different occasions 2
• Serum luteinizing hormone (LH) and follicle-stimulating hormone (FSH) levels to distinguish between primary (hypergonadotropic) and secondary (hypogonadotropic) hypogonadism 2, 3
• Gonadotropin levels to assess the adequacy of androgen replacement therapy 3

Interpretation of Lab Results

The interpretation of lab results is crucial in diagnosing and managing low testosterone. For example:

• Low testosterone levels combined with undetectable, low, or normal gonadotropin levels may indicate hypogonadotropic hypogonadism 3
• A linear correlation between levels of FSH and LH and testosterone levels can be observed in both hypogonadotropic hypogonadism and primary hypergonadism 3
• Higher FSH or LH levels may decrease the chance of achieving eugonadism, as reflected by normal testosterone levels 3

Treatment and Monitoring

Treatment and monitoring of low testosterone involve:

• Testosterone replacement therapy to reverse the symptoms of androgen deficiency 2, 4, 5
• Assessment of clinical responses and measurement of serum testosterone levels to confirm an adequate replacement dosage 2
• Serial measurement of bone mineral density during androgen therapy to confirm end-organ effects 2
• Measurement of hematocrit and digital rectal examination with a serum prostate-specific antigen level measurement for prostate cancer screening 2