Free Testosterone Calculator (Vermeulen)
Most Australian labs report total testosterone. The fraction that is actually active depends on how much is bound to SHBG. This calculator applies the Vermeulen (1999) equation, the same one labs use when they print "calculated free testosterone".
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Vermeulen A, Verdonck L, Kaufman JM. J Clin Endocrinol Metab 1999. Association constants: SHBG 1×10⁹ L/mol, albumin 3.6×10⁴ L/mol.
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Create a free accountFrequently asked questions
What is a normal calculated free testosterone?
For adult men, labs using the Vermeulen method commonly quote roughly 150 to 700 pmol/L (about 4.3 to 20 ng/dL), with age-related decline. Reference intervals for women vary widely between labs, so compare to the range printed on your own report.
Why does SHBG matter so much?
SHBG binds testosterone tightly. When SHBG is high (the pill, ageing, thyroid overactivity, liver disease) a normal total testosterone can hide a low free level. When SHBG is low (insulin resistance, PCOS) free testosterone is higher than the total suggests.
Is calculated free testosterone accurate?
It agrees well with equilibrium dialysis, the reference method, and is far more reliable than the direct analogue immunoassays some labs still offer. It assumes a typical albumin binding affinity, so unusual albumin levels shift the result slightly.
When should testosterone be tested?
Between 7 and 10 am, fasting, when well rested and not acutely unwell. Two low results on separate mornings are needed before a diagnosis of low testosterone in men.
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Educational tool. Results depend on the accuracy of the inputs and the formulas cited; they do not replace your laboratory report or your doctor's interpretation.