Testosterone (TRT) Blood Test Terms Glossary

The nine terms defined

Reference ranges differ between laboratories and assays, so always read your result against the range printed on your own report. The figures below are typical adult-male intervals for orientation only, not diagnostic cut-offs. Diagnosis of hypogonadism requires symptoms plus a repeated, fasting, early-morning total testosterone, per the Endocrine Society guideline.

The first four rows are the inputs to a calculated free-testosterone result. The remaining five are read separately: LH and FSH locate the cause of low T, estradiol balances mood and joint health against water retention, and hematocrit and PSA are the two numbers a clinician watches to keep Testosterone (TRT) safe.

Total vs free testosterone

Picture total testosterone as every passenger on a train. Most are strapped into seats: roughly 44-65% is bound tightly to SHBG and another 33-54% is bound loosely to albumin. Only about 1-2% is walking freely between carriages, and that free fraction is what enters cells and produces an effect. Two men can share an identical total testosterone of 20 nmol/L yet have very different free levels if their SHBG differs, which is why a normal-looking total can still sit alongside low-T symptoms.

Because direct free-testosterone assays (equilibrium dialysis) are expensive and not always offered, the Vermeulen equation estimates free testosterone from total T, SHBG and albumin. The free testosterone index calculator runs that arithmetic for you, and the worked examples below show the conversions it relies on.

How free testosterone is calculated

Calculated free testosterone is not magic; it is unit conversion plus the law of mass action. The honest arithmetic has three stages: put total testosterone into mol/L, put SHBG and albumin into mol/L, then solve the binding equilibrium. You will rarely do the full quadratic by hand, but every input must be in the same unit system first, and that is where most errors creep in.

The safety and source markers

LH and FSH answer one question: is the signal from the brain intact? In primary hypogonadism the testes underperform, so the pituitary shouts louder and LH/FSH run high. In secondary hypogonadism the pituitary signal itself is low, so LH/FSH are low or normal alongside low testosterone. Exogenous Testosterone (TRT) suppresses both, which is why men on testosterone usually show LH and FSH near the floor.

Estradiol matters because testosterone aromatizes into it; too little brings joint aches and low libido, too much brings water retention. Hematocrit is the headline safety number — testosterone stimulates red-cell production, and a rising hematocrit thickens the blood. PSA is tracked because testosterone can unmask existing prostate disease, though most elevated PSA results are benign. None of these are dosing instructions; they are the dashboard a prescriber reads alongside your symptoms.

So, what do the testosterone blood test terms mean?

Each marker on a TRT blood test has a distinct job: total testosterone counts every molecule in the blood, free testosterone reveals the active fraction (roughly 1-2%), SHBG and albumin are the binding proteins that control how much of that total is free, LH and FSH locate the source of any problem, and estradiol, hematocrit, and PSA are the three safety markers monitored throughout treatment. Reading them together — rather than fixating on one number — gives you and your prescriber the full picture. To estimate your own free testosterone from total T, SHBG, and albumin, use the free testosterone index calculator.

Frequently asked questions