From the Guidelines
To evaluate suspected low testosterone, the primary test ordered is total testosterone, ideally measured in the morning when levels are highest, with a cut-off value of below 300 ng/dL, as recommended by the American Urological Association guideline 1. When evaluating low testosterone, it is essential to consider the clinical diagnosis, which requires both a low testosterone measurement and the presence of select symptoms and/or signs. The diagnosis of testosterone deficiency should be made only after two total testosterone measurements are taken on separate occasions, with both conducted in an early morning fashion, as stated in the guideline 1. Additional tests that may be ordered include:
- Free testosterone, which measures the biologically active hormone not bound to proteins
- Bioavailable testosterone
- Sex hormone-binding globulin (SHBG) to determine how much testosterone is bound versus free
- Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) to distinguish between primary hypogonadism (testicular failure) and secondary hypogonadism (pituitary/hypothalamic issues)
- Complete blood count, comprehensive metabolic panel, and prolactin levels to identify other conditions that might affect testosterone
- If secondary hypogonadism is suspected, additional pituitary function tests may be ordered, including thyroid-stimulating hormone (TSH) and morning cortisol, as suggested by the HIV Medicine Association of the Infectious Diseases Society of America 1 and the American Diabetes Association 1. It is crucial to note that patients should avoid biotin supplements for at least 72 hours before testing to prevent interference with test results. The clinical evaluation should also include a targeted physical exam to examine patients for signs associated with low testosterone, such as reduced body hair, gynecomastia, and testicular atrophy, as recommended by the American Urological Association guideline 1.
From the Research
Laboratory Tests for Low Testosterone
To evaluate suspected low testosterone, the following laboratory tests are ordered:
- Total testosterone level, which should be measured in the morning, using a reliable assay, such as liquid chromatography-tandem mass spectrometry, in a laboratory certified by an accuracy-based program 2
- Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) levels to determine whether the hypogonadism is primary or secondary 3
- Prolactin level, which is always necessary in cases of secondary hypogonadism 3
- Free testosterone level, which can be measured by equilibrium dialysis method, especially if total testosterone is near the lower limit of normal or if sex hormone-binding globulin (SHBG) abnormality is suspected 2, 4
- Thyroid function tests, as hypogonadism can be associated with thyroid dysfunction 3
Additional Tests
In certain clinical scenarios, additional tests may be ordered, such as:
- Pituitary hormone levels, including adrenocorticotropic hormone (ACTH), growth hormone, and thyroid-stimulating hormone (TSH) 3
- Pituitary imaging, such as magnetic resonance imaging (MRI) or computed tomography (CT) scan, to evaluate pituitary function and structure 3
- Bone mineral density test, to assess the risk of osteoporosis 4, 5
Timing of Tests
It is recommended to measure testosterone levels in the morning, between 7 am and 12 pm, as testosterone levels can vary throughout the day 6. However, compliance with this recommendation is often low, and further education of practitioners is required to appropriately test patients for hypogonadism 6.