Free testosterone calculator

The formula

Free testosterone (FT) solves the binding equilibrium between testosterone, SHBG and albumin:

N  = 1 + Ka x [albumin]
FT = ( -b + sqrt( b^2 + 4 x N x Kt x [TT] ) ) / ( 2 x N x Kt )
b  = N + Kt x ( [SHBG] - [TT] )

Ka = 3.6 x 10^4 L/mol   (albumin association constant)
Kt = 1.0 x 10^9 L/mol   (SHBG association constant)
albumin molar mass = 69,000 g/mol; all concentrations in mol/L
1 nmol/L total T = 28.84 ng/dL; 1 nmol/L free T = 288.4 pg/mL

Citation: Vermeulen A, Verdonck L, Kaufman JM. A critical evaluation of simple methods for the estimation of free testosterone in serum. J Clin Endocrinol Metab 1999;84(10):3666-72. doi:10.1210/jcem.84.10.6079

When to use it

• Your total testosterone is borderline, or near the bottom of the lab range.
• You have a reason for SHBG to be unusual: ageing, obesity, type 2 diabetes, thyroid or liver problems.
• You want the free T picture without paying for the direct test.

Ask the lab to run total T and SHBG on the same morning sample. SHBG prices vary by branch, so call ahead. See which tests to ask for and normal testosterone levels.