From the Guidelines
Total testosterone is considered the primary measurement in evaluating testosterone deficiency, with a cut-off value of below 300 ng/dL, as recommended by the American Urological Association guideline 1. When evaluating testosterone deficiency, both total and free testosterone levels are important, but they serve different purposes. Total testosterone is typically measured first as the initial screening test, with normal levels generally above 300 ng/dL for adult men, as stated in the AUA guideline 1. If total testosterone is low or borderline, or if conditions affecting sex hormone binding globulin (SHBG) are present, free testosterone should be measured, as suggested by a study on pharmacotherapy for obesity 1. Some key points to consider when evaluating testosterone deficiency include:
- Total testosterone levels should be measured on two separate occasions, in the early morning, to confirm the diagnosis of low testosterone 1.
- Free testosterone represents the biologically active portion (1-2% of total) that is available to bind to receptors in tissues, making it particularly valuable in cases where SHBG levels may be altered 1.
- Conditions like obesity, diabetes, liver disease, and thyroid disorders can affect SHBG levels, causing discrepancies between total and free testosterone measurements 1.
- The calculated free testosterone (using total testosterone and SHBG levels) or direct measurement by equilibrium dialysis are preferred methods for assessment 1.
- For comprehensive evaluation, clinicians should consider both measurements along with clinical symptoms such as decreased libido, erectile dysfunction, fatigue, and reduced muscle mass before making a diagnosis of testosterone deficiency 1.
From the FDA Drug Label
Testosterone in plasma is 98 percent bound to a specific testosterone-estradiol binding globulin, and about 2 percent is free. Generally, the amount of this sex-hormone binding globulin in the plasma will determine the distribution of testosterone between free and bound forms, and the free testosterone concentration will determine its half-life
Free testosterone is considered in evaluating testosterone deficiency, as it is the active form of testosterone that determines its half-life.
- The drug label mentions that only about 2 percent of testosterone in plasma is free, and this free testosterone concentration determines its half-life.
- The label does not explicitly state which form of testosterone is used to evaluate deficiency, but based on the information provided, free testosterone appears to be the relevant form 2
From the Research
Testosterone Measurement
In evaluating testosterone deficiency, both free and total testosterone are considered important.
- Total testosterone is the most commonly measured form, and its levels can be affected by binding proteins such as sex hormone-binding globulin (SHBG) and albumin 3, 4, 5.
- Free testosterone, on the other hand, is the metabolically active fraction and is believed to be the most important form for evaluating androgen deficiency in men 3, 5, 6.
Clinical Utility
The measurement of free testosterone is important in the diagnosis of many diseases, including disorders of androgen deficiency in men (i.e., hypogonadism) and androgen excess in women (i.e., polycystic ovary syndrome and hirsutism) 3.
- The evaluation and management of male hypogonadism should be based on symptoms and on serum testosterone levels, with accurate testing and reference values being crucial for diagnosis 4, 5, 6.
- Total testosterone should ideally be measured in a fasting state in the morning using a reliable assay, such as liquid chromatography tandem mass spectrometry, in a laboratory that is certified by an accuracy-based benchmark 5, 6.
Measurement Methods
Various methodologies are available for measuring free and total testosterone, including:
- Equilibrium dialysis and ultrafiltration for free testosterone measurement 3, 6
- Analog immunoassay and calculated free testosterone based on measurement of total testosterone, SHBG, and albumin 3
- Liquid chromatography-tandem mass spectrometry for total testosterone measurement 5, 6
- Selective protein precipitation or calculation for bioavailable testosterone measurement 3