The fraction of testosterone unbound to SHBG — what tissues actually use. Often more informative than total.
Free testosterone is the fraction not bound to SHBG (sex hormone binding globulin) or albumin. Because SHBG can fluctuate substantially with insulin resistance, alcohol, thyroid status, and obesity, two men with identical total testosterone can have very different free testosterone values — and free testosterone is what actually signals at the androgen receptor.
Free testosterone should always be interpreted alongside SHBG. A man with total T = 700 but SHBG = 85 may have free T below optimal range, while a man with total T = 550 and SHBG = 30 may have free T that's clinically optimal.
Assay method matters: equilibrium dialysis (LC-MS/MS) is the gold standard. Direct immunoassays are less reliable. If the assay isn't specified on the report, look it up.
Free testosterone is influenced by: total testosterone production, SHBG level (higher SHBG = lower free T for same total), insulin resistance (tends to lower SHBG, raising free T modestly), alcohol (raises SHBG), thyroid status, and pharmacologic agents — TRT, HCG, enclomiphene raise it; aromatase inhibitors indirectly shift it.
On TRT, target free testosterone in the upper-optimal range — many men feel best at 18–24 ng/dL. Pair with total T, SHBG, and sensitive estradiol for a complete picture. For enclomiphene monotherapy, expect free T to rise in proportion to total T as the HPG axis restores function.
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Start tracking →Informational only — not medical advice. Reference ranges vary by lab and individual context. Work with a licensed provider to interpret your specific results.