Free testosterone

Free testosterone is the 1-4% of circulating testosterone not bound to SHBG or albumin and represents the immediately bioactive fraction. The gold-standard analytical method is equilibrium dialysis followed by LC-MS/MS; the most validated alternative is the calculated free testosterone (cFT) derived from total testosterone, SHBG and albumin using the Vermeulen equation. Direct analogue immunoassays are inaccurate and not recommended by the Endocrine Society. According to the 2018 Endocrine Society guideline, a calculated free testosterone below approximately 65 pg/mL (220 pmol/L) in symptomatic men supports a diagnosis of androgen deficiency, with the lower limit of normal in healthy young men typically around 70 pg/mL (~7 ng/dL, 0.24 nmol/L). Confounders include SHBG-altering states (insulin resistance, thyroid disease, oral estrogens), morning vs afternoon sampling, acute illness, and inter-assay differences in SHBG that propagate into the calculation.