Free Testosterone Calculator
Only about 1 to 3% of your total testosterone circulates in its free, unbound form. The rest is bound to SHBG or albumin. This calculator uses the Vermeulen method to estimate your free and bioavailable testosterone from standard lab values.
Testosterone in the blood exists in three forms: tightly bound to sex hormone-binding globulin (SHBG), loosely bound to albumin, and free (unbound). SHBG-bound testosterone is generally considered inactive because the binding is too tight for tissues to use. Albumin-bound testosterone can dissociate easily and is considered bioavailable along with the free fraction.
The Vermeulen method calculates free testosterone using the law of mass action, factoring in the binding affinities of SHBG and albumin. It solves a quadratic equation based on total testosterone, SHBG concentration, and albumin level. This approach is more accurate than simple ratio methods (like total T divided by SHBG) and is widely used in clinical research.
Free testosterone is clinically important because some patients have normal total testosterone but low free testosterone due to elevated SHBG. SHBG rises with age, liver disease, hyperthyroidism, and estrogen use. It decreases with obesity, hypothyroidism, and androgen use. Measuring or calculating free testosterone gives a clearer picture of androgen status than total testosterone alone.