Total testosterone is the number most men are given. It counts every testosterone molecule in the sample, including the large fraction that is bound up and unavailable to tissue. That distinction explains many confusing results.
The three fractions
Roughly 40 to 60 percent of circulating testosterone is tightly bound to sex hormone-binding globulin (SHBG). Most of the remainder is loosely bound to albumin, and only 1 to 3 percent circulates free.
Free testosterone plus the albumin-bound fraction is often described as bioavailable testosterone, because both can reach tissue relatively easily.
What moves SHBG
SHBG is not fixed. When it rises, more testosterone is bound and total testosterone can look normal while the usable fraction is low. When it falls, the opposite happens.
- SHBG rises with age, hyperthyroidism, liver disease, some anticonvulsants and significant calorie restriction.
- SHBG falls with obesity, insulin resistance, type 2 diabetes, hypothyroidism, corticosteroids and nephrotic syndrome.
How the numbers are read together
A man with high SHBG can have a reassuring total testosterone and genuinely low free testosterone. A man with obesity and low SHBG can have a low total testosterone with relatively preserved free testosterone.
That is why guidelines recommend measuring free testosterone (by equilibrium dialysis or a validated calculation using total testosterone, SHBG and albumin) when the total result and the symptoms disagree, or when a condition that alters SHBG is present.
Practical measurement rules
Testing conditions matter more than most men are told.
- Sample in the morning, ideally before 10 a.m., after an overnight fast.
- Repeat any abnormal result on a second morning before drawing conclusions.
- Avoid testing during acute illness or immediately after extreme training or sleep loss.
- Direct free-testosterone immunoassays are considered unreliable; calculated or dialysis methods are preferred.
Key takeaways
- Only a small fraction of testosterone is free and immediately available to tissue.
- SHBG changes with age, weight, thyroid status, liver health and medications.
- Normal total testosterone with high SHBG can still mean low free testosterone.
- Calculated or dialysis free testosterone is preferred over direct immunoassays.
- Always sample in the morning, fasted, and repeat abnormal results.
References
- Endocrine Society. Testosterone Therapy in Men With Hypogonadism: Clinical Practice Guideline
- American Urological Association. Testosterone Deficiency Guideline: laboratory evaluation
How this article is reviewed
Aldrick articles are written from published guidance and peer-reviewed research, checked against our evidence standards and re-reviewed when guidance changes. Spotted something inaccurate or out of date? Tell us and we will correct it and update the review date.
This article is educational. It does not diagnose conditions or replace evaluation by a qualified clinician.