Free Testosterone: Normal Range and Why It Matters When SHBG Is Abnormal
Free testosterone measures the 1–3% of testosterone not bound to proteins, which is the fraction available to tissues. Normal is roughly 50–210 pg/mL in adult men. It is the more informative test whenever SHBG is abnormal — in obesity, ageing, thyroid disease, liver disease or on oestrogen therapy.
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Detailed Clinical Overview
Testosterone circulates in three states: tightly bound to sex hormone binding globulin and biologically unavailable, loosely bound to albumin and partly available, and unbound and fully active. Free testosterone measures only the last of these. Bioavailable testosterone, a related measure, includes the albumin-bound fraction as well.
The clinical need for it arises entirely from SHBG. A man with obesity and insulin resistance may have a low SHBG and therefore a low total testosterone while his free testosterone is perfectly normal; an older man with a high SHBG may have a normal-looking total testosterone while his free level is genuinely low. In both cases the total misleads and the free does not.
How it is measured matters. Direct analogue free testosterone immunoassays are widely available but poorly reliable. The recommended approach is calculated free testosterone, derived from total testosterone, SHBG and albumin using a validated equation, or equilibrium dialysis where the highest accuracy is required.
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Frequently Asked Questions
Q: What is the normal free testosterone level?
A: Roughly 50 to 210 pg/mL in adult men and 1.0 to 8.5 pg/mL in adult women, though intervals differ substantially between methods. Always read your result against the interval printed on your own report rather than a general figure.
Q: When is free testosterone better than total testosterone?
A: Whenever SHBG is abnormal — in obesity, type 2 diabetes, ageing, thyroid disease, liver disease, nephrotic syndrome, or on oestrogen or anticonvulsant therapy. In these situations the total testosterone can be misleadingly high or low while the free level reflects what tissues actually receive.
Q: What is calculated free testosterone?
A: It is free testosterone derived mathematically from total testosterone, SHBG and albumin using a validated equation, rather than measured directly. It is preferred over direct free testosterone immunoassays, which are widely available but known to be unreliable, particularly at low concentrations.
Q: How does SHBG affect testosterone results?
A: SHBG binds testosterone tightly and makes it unavailable to tissues. A low SHBG, common in obesity and insulin resistance, lowers total testosterone while free testosterone stays normal. A high SHBG, common with ageing and hyperthyroidism, keeps total testosterone looking normal while free testosterone is genuinely low.
Q: What is the free androgen index?
A: It is total testosterone divided by SHBG, multiplied by 100 — a simple estimate of androgen availability used mainly in women. A value below 5 is normal, and a raised index supports androgen excess in PCOS and hirsutism, often when total testosterone alone reads normal.
Q: Do I need a morning sample for free testosterone?
A: Yes. Free testosterone follows the same daily rhythm as total testosterone, peaking between 7 and 10 a.m. An afternoon sample can read substantially lower and is a common cause of a spurious diagnosis of deficiency.
Q: Which is better — free testosterone or bioavailable testosterone?
A: Both are reasonable. Free testosterone measures only the unbound fraction; bioavailable testosterone includes the loosely albumin-bound fraction as well, on the basis that it is partly accessible to tissues. In practice, calculated free testosterone is more widely used and adequately reflects androgen status.
Q: Should free testosterone be tested in women with PCOS?
A: It is often more informative than total testosterone. Many women with PCOS have a low SHBG driven by insulin resistance, so free testosterone or the free androgen index is raised while total testosterone still reads normal. It should be measured in the early follicular phase where cycles are regular.
Q: Can free testosterone be normal when total is low?
A: Yes, and it is a common scenario in obesity. A low SHBG lowers total testosterone without reducing the biologically available amount, so the man is not truly androgen deficient. Diagnosing hypogonadism on the total alone in that situation leads to unnecessary treatment.
Q: What other tests should accompany free testosterone?
A: Total testosterone, SHBG and albumin at minimum, since they are needed for the calculation. Depending on the question, LH and FSH to localise a deficiency, prolactin to exclude a pituitary adenoma, and in women DHEAS and 17-hydroxyprogesterone when androgen excess is being investigated.
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