Bloodwork
Two men get their bloodwork back. Both have a total testosterone of 600 ng/dL, comfortably mid-range. One feels fine. The other has no libido, no drive and no explanation.
The difference is usually SHBG, and it is missing from most panels.
Sex hormone binding globulin is a protein made by your liver. Its job is to bind sex hormones and carry them around the bloodstream. Bound testosterone is inactive. It cannot enter a cell, cannot bind a receptor, and cannot do anything at all until it is released. Only the unbound fraction, and a loosely bound portion attached to albumin, is available to tissue.
Here is the part that surprises people: roughly one to two percent of your total testosterone is free. Around 98 percent is bound, most of it tightly to SHBG.
So when a lab reports total testosterone, it is reporting almost entirely hormone that is not currently doing anything. The number is real. It is just not the number that determines how you feel.
Return to the two men. Both at 600 ng/dL total.
The first has SHBG of 25 nmol/L, at the lower end of normal. Relatively little of his testosterone is locked up, so his free testosterone is comfortable. The second has SHBG of 70 nmol/L, at the upper end. Far more of the same total is bound and unavailable, so his free testosterone is meaningfully lower despite an identical headline figure.
Same total. Different available hormone. Different experience.
This is why a total testosterone result on its own cannot be interpreted, and why the bloodwork analyzer calculates free testosterone rather than reading the total in isolation.
Labs can measure free testosterone directly, but the common direct assays are unreliable and most clinicians do not trust them. The standard approach is to calculate it instead, using total testosterone, SHBG and albumin, through an equation published by Vermeulen and colleagues in 1999. It is well validated and it is what most calculators and clinical tools use.
The practical consequence: you need SHBG on the panel or free testosterone cannot be calculated at all. Ordering total testosterone without SHBG produces a number nobody can properly read.
Several of these are modifiable, and one of them catches a lot of people mid-diet.
Caloric restriction. Sustained dieting pushes SHBG up. Someone sixteen weeks into a hard cut often sees SHBG climb and free testosterone fall, then blames the training or the programme. It is the deficit.
Thyroid hormone excess. Hyperthyroidism, or an over-replaced hypothyroid patient, raises SHBG reliably.
Ageing. SHBG rises gradually through adult life, which is part of why free testosterone declines faster with age than total testosterone does.
Liver conditions. SHBG is made in the liver, so liver disease affects production.
Oestrogen. Including some medications.
Anticonvulsants and certain other drugs.
Insulin resistance. The strongest and most common cause of genuinely low SHBG. Low SHBG is well established as a marker of metabolic dysfunction, and it often appears before fasting glucose moves.
Excess body fat, particularly visceral fat, through the same mechanism.
Hypothyroidism.
Androgens, including exogenous testosterone.
Growth hormone.
This gets misread constantly. Low SHBG means more of your testosterone is free, so free testosterone looks good on paper. But low SHBG is also a recognised signal of insulin resistance. A man with low SHBG, good free testosterone and a rising waistline has a metabolic problem, not a hormonal advantage.
If your SHBG is low, the useful next step is looking at fasting glucose, HbA1c and fasting insulin rather than congratulating yourself on the free testosterone figure. The Insulin Resistance Reversal System goes through exactly that bloodwork, how to grade the severity, and how to fix it.
Partly genetic, partly modifiable, and the modifiable part is larger than most people assume.
Check your thyroid. Hyperthyroidism or over-replacement is a common and correctable cause.
Look at how long you have been dieting. If you have been in a deficit for months, that alone can explain it. A maintenance phase often moves the number.
Check liver markers. ALT, AST and GGT, alongside your alcohol intake.
Review medications with whoever prescribed them.
If thyroid, liver and energy availability are all clean and SHBG is still high, it is probably yours to keep. In that case the answer is not to chase the SHBG number but to read free testosterone instead of total, and judge the situation on that.
For any question about testosterone, the minimum useful set is:
Standard full-body packages almost never include SHBG. It usually has to be requested by name, and it typically costs very little on its own. The blood test selector flags exactly which markers packages leave out.
Total testosterone tells you how much hormone is in circulation. SHBG tells you how much of it you can actually use. One without the other is half a result, and it is the half that explains why two men with the same number feel completely different.
If you have ever had a testosterone result that looked normal while you felt nothing like it, this is the first place to look.
Roughly 10 to 57 nmol/L in adult men, though reference ranges vary between laboratories. Compare against the range printed on your own report rather than one found online.
More of your testosterone is bound and unavailable, so free testosterone is lower than the total suggests. Common causes include sustained caloric restriction, thyroid hormone excess, ageing and liver conditions.
More testosterone is free, which looks favourable, but low SHBG is a recognised marker of insulin resistance. It usually warrants checking fasting glucose, HbA1c and insulin rather than being treated as good news.
Sometimes. If it is raised by a prolonged calorie deficit, over-replaced thyroid medication or high alcohol intake, addressing those can move it. If those are all normal, SHBG is largely genetic and the better approach is to read free testosterone instead.
Most standard full-body packages exclude it, because they are built for breadth across a general population rather than depth on a specific question. It usually has to be requested by name and costs very little.
For explaining symptoms, generally yes, because only the free and loosely bound fraction is available to tissue. Both are useful together, which is why the calculation needs total, SHBG and albumin.
Vermeulen A, Verdonck L, Kaufman JM. A critical evaluation of simple methods for the estimation of free testosterone in serum. J Clin Endocrinol Metab. 1999;84(10):3666–3672. Read the paper
This article is educational and is not medical advice. See the medical disclaimer.
Want a programme built around your own numbers?
Work With Dinesh
Newsletter
One email when a new guide, calculator or breakdown goes live. Nothing else.
