SHBG: the missing marker that decides whether your testosterone number means anything

Why SHBG matters more than you'd think

Sex hormone-binding globulin (SHBG) is a glycoprotein made by the liver. Its job in circulation is to bind sex steroids — primarily testosterone, dihydrotestosterone, and oestradiol — and reduce their bioavailability. Roughly 98% of your total testosterone is bound: most to SHBG (high affinity), some to albumin (low affinity, weak grip). Only the small remaining fraction is "free" — directly available to receptors.

That sentence is the entire reason this marker matters. Two men can have an identical total testosterone reading, say 550 ng/dL — comfortably in the middle of the range — and have very different free-T values depending on their SHBG. One feels fine. The other has low libido, slow recovery, and a creeping fatigue that no GP has been able to explain.

The difference between those two men isn't in the testosterone number. It's in the SHBG.

The arithmetic, briefly

Free testosterone is calculated from total testosterone, SHBG and albumin using a standard formula (the Vermeulen calculation is the most widely cited). The relationship is non-linear: at low SHBG, small changes in SHBG produce large changes in calculated free T. At high SHBG, the curve flattens.

What this means in practice:

• A man with total T = 550 ng/dL and SHBG = 25 nmol/L (low-normal) has calculated free T around 130 pg/mL. Adequate.
• A man with total T = 550 ng/dL and SHBG = 65 nmol/L (high) has calculated free T around 75 pg/mL. Often symptomatic.
• A man with total T = 320 ng/dL and SHBG = 15 nmol/L (low) has calculated free T around 80 pg/mL. Approaches functional adequacy despite a "low" total T.

Same total T values can be very different stories. Different total T values can be very similar stories. The marker that bridges them is SHBG.

Why high SHBG is more common than people think

Most clinical attention goes to low SHBG, because of its tight correlation with insulin resistance. But high SHBG is what we see in a different population: lean, restricted-eating, often endurance-trained men whose total T looks normal and whose free T is quietly inadequate.

The drivers of high SHBG worth knowing:

• Energy deficit. Calorie restriction below maintenance for months raises SHBG; this is partly why "I trained hard, ate clean, and still feel terrible" is a hormone story before it's a programming story.
• Hyperthyroidism. Increased thyroid hormone exposure raises SHBG production. If high SHBG appears without obvious cause, a TSH check belongs next.
• Liver dysfunction. Particularly the early stages of alcohol-related liver change, before traditional markers move.
• Aging. SHBG slowly rises with age, independent of total T. Hence the textbook pattern of "total T flat with age, free T declines."
• Anti-androgens and certain medications. Including some anticonvulsants and many anti-androgen agents.

A common case: a 35-year-old endurance runner, 70 kg, training six days a week with disciplined nutrition. Energy is low; libido is dropping; he doesn't sleep as well as he used to. Total T comes back at 580 — "fine." SHBG comes back at 72. Free T is calculated at 70 pg/mL — low. The intervention here is rarely testosterone. It is restoring energy availability — more food, less training stress, ideally a deload — over weeks. SHBG drops, free T rises, symptoms resolve.

Without measuring SHBG, this man is told he is fine. He is not fine.

Why low SHBG matters even more

The clinical signal that low SHBG carries is striking. In large cohort studies, low SHBG predicts incident type 2 diabetes and metabolic syndrome better than fasting glucose or HbA1c in the years before either of those move. It is one of the earliest visible hormonal traces of the insulin-resistance trajectory.

The mechanism is well-understood: insulin suppresses hepatic SHBG synthesis. The more insulin you produce on average (driven by visceral adiposity, refined-carbohydrate dominance, sedentary baseline), the lower your SHBG. By the time HbA1c drifts up, the SHBG has been quietly dropping for years.

That means a young, ostensibly fit man with low SHBG, even with a "normal" total T, is sometimes the most important kind of finding on a panel. The marker tells you something about his metabolic trajectory that no other line on the panel does — and gives you a years-of-runway head start to do something about it.

How SHBG reads alongside the rest of the panel

On the Hormone Panel 01, SHBG is the bridge between total testosterone and the free fraction. It also reads against:

• Estradiol. SHBG binds E2 too. Very high SHBG can leave free E2 inadequate even with total E2 in range.
• LH and FSH. A man with high SHBG and "fine" total T but functionally low free T may show normal LH and FSH on a single draw — because the brain reads the (much smaller) free fraction less directly than you might assume.
• Thyroid. Not measured on this panel, but inferable: unexplained high SHBG warrants a TSH check; unexplained low SHBG warrants an HbA1c and a fasting insulin.

The two key questions to ask the panel:

1. Does my SHBG explain my symptoms in spite of a normal-looking total T?
2. Does my SHBG tell me something about my metabolic trajectory that my total T doesn't?

For many men, the answer to one of those is yes. For some, both.

What to do with the number

If your SHBG is high (above ~55 nmol/L in a younger man) and your free T is calculated as low or low-normal:

• Look at energy availability first. Are you eating enough relative to training stress?
• Check thyroid (TSH, fT4) with your GP.
• Consider alcohol intake honestly.

If your SHBG is low (below ~20 nmol/L) and you're not on exogenous androgens:

• This is one of the strongest early signals of insulin resistance. Check fasting glucose and HbA1c next.
• Body composition and aerobic capacity work — boring, but it's what moves SHBG back up over months.
• Don't add testosterone on top. Exogenous androgens lower SHBG further; the problem compounds.

If your SHBG is mid-range and your total T is the lever you're looking at:

• This is the simple read. The total T number means what it appears to mean. Symptoms that don't track with the number are not a hormone story.

The marker is quietly the most informative number on the panel for a large minority of men. It's also the cheapest part of the panel to add to a standard testosterone test. It is missed mostly because nobody knows what to do with it.

Now you do.

Sources cited: Endocrine Society Clinical Practice Guideline on Testosterone Therapy in Men with Hypogonadism, 2018; full entry on /science.