Free testosterone

Women: 3 to 30 pmol/L · Men: 200 to 620 pmol/L

Most UK labs calculate it from total testosterone, SHBG and albumin rather than measuring it, and the range depends on the formula used, so ranges vary widely (men roughly 200 to 620 pmol/L, women below about 30).

Only a small fraction of testosterone, around two per cent, travels unbound in the blood; the rest is attached to SHBG and albumin. Free testosterone is an estimate of that unbound, active part. In the UK it is nearly always calculated from the total testosterone, SHBG and albumin rather than measured directly.

Doctors use it when total testosterone is near the lower end of the range or when SHBG is unusual, for example in heavier men, older men, people with liver or thyroid conditions, or women being assessed for polycystic ovary syndrome, because SHBG changes the total without changing the active part.

• Testosterone replacement and anabolic steroid use
• Low SHBG, for example with heavier body weight, type 2 diabetes or an underactive thyroid
• Polycystic ovary syndrome, in women
• Morning sampling, as with total testosterone

• Older age, in the general population
• High SHBG, for example with an overactive thyroid, liver disease, the combined pill or HRT
• Testicular, pituitary or hypothalamus problems
• Afternoon sampling, acute illness and poor sleep
• Some medicines, including opioids and steroids

• Different calculation formulas give different numbers from the same blood sample, so results from two labs are not directly comparable
• It inherits all the day-to-day variation of total testosterone, including time of day
• Direct 'free testosterone' immunoassays used by some private labs are known to be unreliable

Same as total testosterone: a morning sample before 11am, ideally fasting, repeated once if low. It is only as good as the total testosterone and SHBG it is calculated from.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.