What Do Total Testosterone, Free Testosterone, and SHBG Tell You?
Why most testosterone is bound to proteins, when free testosterone matters, how to time the test, what a harmonized normal range for men looks like, and why one low result isn't a diagnosis.
By [Author name]4 min read
A testosterone test usually measures total testosterone: everything in your serum, bound and unbound. Most of it is bound to proteins, mainly sex hormone-binding globulin (SHBG) and albumin, and only a small free fraction is readily available to your tissues. So when your SHBG is unusually high or low, total testosterone can mislead, and free testosterone is the better guide. Either way, one result is never enough: the Endocrine Society’s guideline asks for consistently low morning results, together with symptoms, before diagnosing low testosterone.
This article is general information, not medical advice.
Bound and Free
MedlinePlus describes three tests:
- Total testosterone, bound and free together, the most common.
- Free testosterone, only the unbound part, less common but useful for some conditions.
- Bioavailable testosterone, the free part plus what’s loosely bound to proteins other than SHBG, rarely done.
SHBG binds testosterone tightly, so the more SHBG you have, the less of your total is free. Two men with the same total testosterone can have quite different free testosterone if their SHBG differs.
How Free Testosterone Is Measured
The reference method is equilibrium dialysis, which physically separates free from bound hormone, but it’s slow and done by fewer labs. The common alternative is to calculate free testosterone from total testosterone, SHBG, and albumin. In their 1999 comparison, Vermeulen and colleagues found that calculated free testosterone was almost identical to equilibrium dialysis in every condition they studied except pregnancy, while direct immunoassays of free testosterone with a labelled analogue, and the free androgen index (total testosterone divided by SHBG), weren’t reliable.
The Endocrine Society’s 2018 guideline (Bhasin and colleagues) recommends free testosterone, by equilibrium dialysis or an accurate formula, when total testosterone is near the lower limit of normal or when you have a condition that alters SHBG.
Timing the Test
Testosterone is highest in the morning and varies from day to day. MedlinePlus says the sample is usually taken between 7 and 10 a.m., and the Endocrine Society recommends measuring fasting morning total testosterone with an accurate assay, then repeating it to confirm a low result.
What’s Normal for Men
Normal ranges have differed between labs, partly because assays differ. Travison and colleagues harmonized measurements from four large cohorts in the United States and Europe against the CDC’s reference method. In healthy, non-obese men aged 19 to 39, the harmonized range (2.5th to 97.5th percentile) was 264 to 916 ng/dL, with a median of 531 ng/dL. At every age, levels in non-obese men were higher than in the cohorts’ men as a whole.
A result below the range isn’t a diagnosis by itself. The Endocrine Society recommends diagnosing hypogonadism only in men with symptoms and signs of testosterone deficiency and unequivocally, consistently low testosterone.
Women
Women have far lower testosterone, and their labs print a different range. MedlinePlus lists high testosterone in women among the signs of conditions such as polycystic ovary syndrome (PCOS), or adrenal or ovarian disorders.
Following It in Kaizora
Serum Total Testosterone, Sex Hormone-Binding Globulin, Serum Free Testosterone (Calculated), and Serum Free Testosterone (Equilibrium Dialysis) are lab results on Labs, each logged with its time, laboratory, and the range it printed, so a result outside that range is flagged. The two free testosterone methods are separate metrics, so a change of method never looks like a change in you. A draw’s results share their time and laboratory, so your total testosterone and SHBG from the same morning line up. Testosterone has no default range in Kaizora, since your lab prints its own range on each result.
The Short Version
- Total testosterone counts bound and free; free testosterone is what tissues readily use.
- When SHBG is unusual or total testosterone is borderline, free testosterone, by equilibrium dialysis or calculation, is the better guide.
- Test fasting in the morning, and repeat a low result.
- A harmonized normal range for healthy young men is about 264 to 916 ng/dL; a diagnosis also needs symptoms.
Sources
- Bhasin S et al., “Testosterone therapy in men with hypogonadism: an Endocrine Society clinical practice guideline,” Journal of Clinical Endocrinology & Metabolism 103, no. 5 (2018): 1715–1744. doi:10.1210/jc.2018-00229
- Travison TG et al., “Harmonized reference ranges for circulating testosterone levels in men of four cohort studies in the United States and Europe,” Journal of Clinical Endocrinology & Metabolism 102, no. 4 (2017): 1161–1173. doi:10.1210/jc.2016-2935
- Vermeulen A, Verdonck L, Kaufman JM, “A critical evaluation of simple methods for the estimation of free testosterone in serum,” Journal of Clinical Endocrinology & Metabolism 84, no. 10 (1999): 3666–3672. doi:10.1210/jcem.84.10.6079
- MedlinePlus, Testosterone levels test.
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