Key takeaways
- Total testosterone measures everything in blood, about 50-60% bound to SHBG (inactive), 40% loosely bound to albumin, and 1-3% free.
- The clinically relevant fractions are free and bioavailable T.
- Because SHBG decides how much testosterone is bound, anything that changes SHBG changes your free testosterone without changing your total at all.
- Total testosterone measures the reservoir; free and bioavailable testosterone measure what tissue can reach.
Two men walk out of the same lab with a total testosterone of 400 ng/dL. One feels fine. The other has been exhausted for two years, has lost his drive, and has just been told his levels are normal. Both results are accurate. The difference between them is not in the number they were given — it is in a number neither of them was told to order.
Testosterone travels in three states
Testosterone does not circulate freely in the bloodstream. Almost all of it is bound to carrier proteins, and how tightly it is bound determines whether tissue can use it:
- SHBG-bound (roughly 50-60%). Sex hormone-binding globulin grips testosterone tightly. This fraction is effectively locked away and biologically inactive.
- Albumin-bound (roughly 40-45%). A loose, low-affinity attachment. It dissociates readily at the capillary, so this fraction is generally treated as available.
- Free (roughly 1-3%). Unbound and immediately able to enter cells and bind receptors.
Add the free and albumin-bound portions together and you get bioavailable testosterone — around 45% of the total, and the closest single description of what your body can actually access.
What each number is telling you
Total testosterone is the size of the reservoir. It says how much is in circulation, and nothing about how much is usable.
Free testosterone is the fraction available right now. It is the number that tracks symptoms most closely, and the one most often missing from a standard panel.
Bioavailable testosterone sits between the two and is the most physiologically honest of the three, though it is ordered less often.
Ordering total alone is like being told the balance of an account without being told how much of it is locked in a fixed-term deposit. The headline figure is real. It just does not answer the question you asked.
SHBG is the variable that explains the gap
Because SHBG decides how much testosterone is bound, anything that changes SHBG changes your free testosterone without changing your total at all. And SHBG moves for plenty of reasons:
- SHBG rises with age, with hyperthyroidism, with liver disease, with oestrogen exposure, and in some men with very low body fat or prolonged energy restriction.
- SHBG falls with insulin resistance and obesity, in hypothyroidism, with significant alcohol-related liver change, and with corticosteroid or androgen exposure.
This is why SHBG belongs on the panel rather than being treated as an obscure extra. Without it, a total testosterone result cannot be interpreted properly, and a calculated free testosterone cannot be produced at all.
The patterns where total and free diverge
Three presentations account for most of the confusion:
- The older man with high SHBG. Total testosterone reads 400-500 and looks unremarkable. SHBG is elevated, free testosterone is low, and he has every symptom in the book. This is the classic "your labs are normal" case, and the free value is what exposes it.
- The insulin-resistant man with low SHBG. Total testosterone reads 350 and looks borderline or low. SHBG is suppressed, so a larger share is unbound and free testosterone may sit in the normal range. He is often less symptomatic than the total implies — and the more useful finding is the insulin resistance driving the low SHBG in the first place. See fasting insulin.
- Thyroid dysfunction in either direction. Thyroid status moves SHBG substantially, so an untreated thyroid problem can make the testosterone picture look better or worse than it is.
In all three, the total testosterone is not wrong. It is simply insufficient on its own to say what is happening.
Measured versus calculated free testosterone
There are two ways to get a free testosterone value, and they are not equivalent.
Equilibrium dialysis is the reference method. It physically separates the free fraction and measures it. It is accurate, more expensive, and not offered by every laboratory.
Calculated free testosterone derives the value from total testosterone, SHBG and albumin using a published equation — most commonly the one described by Vermeulen et al. (J Clin Endocrinol Metab 1999). It agrees closely with dialysis across most of the range and costs nothing extra if you already have the three inputs. For most people this is the practical choice.
The method to be wary of is the older direct analogue immunoassay for free testosterone. It is cheap, still in use in some labs, and considered unreliable. If a free testosterone result looks wildly out of keeping with the rest of the panel, the assay is worth asking about.
One consequence matters practically: because units and methods differ between laboratories, free testosterone results are not directly comparable across labs. If you are tracking change over time, stay with one laboratory and one method.
How to read your own panel
Work through it in this order rather than starting with the headline:
- Check the draw was morning and fasted. If it was not, the panel may be describing your afternoon rather than you.
- Look at SHBG first. It frames everything that follows.
- Then free testosterone, and note the units and the method — pg/mL and ng/dL are both in use and differ by a factor of ten.
- Then total, as context rather than as the verdict.
- Then LH and FSH, which indicate whether any shortfall originates in the testes or upstream in the pituitary.
- Then estradiol and thyroid, which shape both symptoms and SHBG.
And hold the whole thing against how you actually feel. A number that is unremarkable in a man with no symptoms means something different from the same number in a man who has been struggling for two years.
What the guideline says about targets on treatment
If testosterone therapy is ever prescribed, the target is not "as high as possible." The Endocrine Society clinical practice guideline (Bhasin et al., J Clin Endocrinol Metab 2018) sets the aim as restoring concentrations to the mid-normal range for healthy young men, and it defines monitoring around that — a repeat measurement a few months in, timed consistently relative to the dose, alongside haematocrit and PSA where indicated.
That is the reference to raise with a prescriber rather than a number from an article. Two reasons it matters here: the target is expressed against a laboratory's own reference range, so it is not a single universal figure; and symptom response, not the number on its own, is what the guideline treats as the measure of whether therapy is working.
The clinical pearl: Always order total testosterone, free testosterone and SHBG together. The most common failure in this area is not a wrong result — it is a right result that was never going to answer the question, because two thirds of the panel was missing.
Bottom line
Total testosterone measures the reservoir; free and bioavailable testosterone measure what tissue can reach. SHBG determines the relationship between them and shifts with age, thyroid status and insulin resistance, which is why two men with identical totals can be in completely different situations. Order all three, insist on a morning fasted draw, prefer calculated free testosterone from a reliable lab over an older direct assay, stay with one laboratory when tracking, and interpret the set against your symptoms rather than reading any single line in isolation.
Educational content, not medical advice. Reference ranges, assay methods and units vary between laboratories. Laboratory interpretation and any treatment decision are made by a licensed physician after individual evaluation. Individual results vary.
