SHBG: What It Is and How to Move It

Two men have an identical total testosterone. One feels fine. The other has the full symptom list and has been told, repeatedly, that his level is normal. The variable separating them is usually a protein made by the liver that nobody mentioned, because the panel that was ordered did not include it. Sex hormone binding globulin decides how much hormone reaches tissue, and without it a total testosterone result is a number without a denominator.

What SHBG is, and what it is for

SHBG is a glycoprotein produced almost entirely by the liver, circulating as a dimer with a high-affinity binding site for steroid hormones (Hammond, Biol Reprod 2011). It does two jobs. It acts as a circulating reservoir, buffering the pulsatile output of the gonads into something steadier. And it acts as a gatekeeper, because a hormone bound to it cannot enter a cell.

The second job is why it matters clinically. Hepatic SHBG production is not fixed — it is regulated by insulin, thyroid hormone, oral estrogen and several other inputs, which means the gate is constantly being adjusted by the person's metabolic state, usually without anyone noticing.

Binding is selective, and that has consequences

SHBG does not bind all steroids equally: DHT highest, testosterone high, estradiol moderate and meaningfully weaker, DHEA weak.

That gap between testosterone and estradiol is the part that gets missed. Because SHBG grips testosterone more tightly, a rise in SHBG removes proportionally more testosterone than estradiol from the free pool. The result is not simply less free testosterone — it is a shifted balance between free testosterone and free estradiol, a different hormonal environment rather than a diluted version of the same one. It is one reason men with high SHBG can present with symptoms that read as low testosterone and relatively high estrogen at once (estradiol in men).

The fractions, and the limits of the model

• SHBG-bound, around 50-60% — held tightly, functionally unavailable
• Albumin-bound, around 40-45% — bound weakly and released readily during capillary transit
• Free, around 1-3% — immediately available

Bioavailable testosterone is free plus albumin-bound: what tissue can actually access (Vermeulen et al., J Clin Endocrinol Metab 1999). It is worth saying plainly that this free-hormone model is a useful simplification rather than the complete truth — there is evidence that some bound hormone is taken up by tissues through receptor-mediated routes. It is nonetheless the best available framework, and it predicts symptoms far better than total testosterone alone.

How to get free testosterone measured properly

This is where panels go wrong. The cheap direct analogue immunoassay for free testosterone is unreliable and should not be the basis for a decision. The dependable approach is equilibrium dialysis, or a calculated free testosterone derived from total testosterone, SHBG and albumin — accurate enough for clinical use, and requiring only that SHBG was actually ordered. Ask for all three on the same morning draw and let the free value be calculated. Free versus total testosterone and SHBG explained cover the arithmetic.

What raises SHBG

• Ageing — a slow, steady rise across adult life, which is why an unchanged total testosterone at 55 does not mean an unchanged hormonal position (Vermeulen, biological indexes and SHBG in ageing men)
• Thyroid hormone excess — it directly upregulates hepatic SHBG transcription
• Oral estrogen, including combined contraceptives and oral hormone therapy
• Energy restriction and very low body fat, including prolonged aggressive dieting
• Chronic alcohol use and liver disease; some medications, notably anticonvulsants

The oral estrogen point carries a decision inside it. Oral estrogen passes through the liver before reaching the circulation, and that first pass is what drives SHBG up sharply; transdermal delivery bypasses it and has a much smaller effect. For a woman whose free testosterone and libido matter to her, route of administration is not a trivial detail.

What lowers SHBG

• Insulin resistance and hyperinsulinaemia — the dominant driver in most adults
• Obesity, particularly visceral adiposity
• Hypothyroidism; androgen excess, including PCOS and exogenous androgens
• Growth hormone excess and cortisol excess

Insulin is the one to understand, because it explains most low-SHBG panels. Insulin suppresses hepatic SHBG gene transcription, shown directly in hepatoma cells (Plymate et al., J Clin Endocrinol Metab 1988), through the same hepatic transcription factors that respond to nutrient load — which is why SHBG also tracks liver fat (Simó et al., Trends Endocrinol Metab).

That makes a low SHBG informative in its own right: an early, sensitive readout of hepatic insulin exposure, often abnormal before fasting glucose is, and independently predictive of later type 2 diabetes in both sexes (Ding et al., NEJM 2009). A man told his low SHBG is good news because his free testosterone looks healthy has been given half the story.

Reading the four common patterns

• Low SHBG, normal total testosterone. Free testosterone may look fine or even high. The finding to act on is the metabolic state that suppressed the SHBG — check fasting insulin and liver markers.
• High SHBG, normal total testosterone. The classic missed diagnosis: free testosterone genuinely low while the total sits mid-range and the patient is told he is normal. Ageing and thyroid excess are the usual causes; a hard diet is a common reversible one.
• Low SHBG, low total testosterone. Metabolic and axis dysfunction together — typically the visceral fat loop. The free fraction is often less deficient than the total suggests.
• High SHBG, low total testosterone. Least common and most functionally deficient, because both terms move the same way.

The HPG axis end to end covers where in the chain these originate.

Moving it, and what to expect

SHBG is a symptom of something else far more often than a target in itself. When it is too low, the driver is usually insulin: visceral fat loss, resistance training, less refined carbohydrate, and metabolic therapy where a physician judges it appropriate all raise it as a by-product, as does correcting untreated hypothyroidism. Expect a slow response — hepatic protein synthesis reflects a sustained change in metabolic state, so three months is the shortest re-test interval that tells you anything.

When it is too high, find the reason before intervening: reducing alcohol, treating hyperthyroidism, easing an over-aggressive deficit and eating adequate protein all bring it down. What should not happen is treating the number while ignoring what moved it — a rising SHBG in a lean, hard-dieting athlete and a rising SHBG in untreated thyroid disease are the same number describing entirely different situations.

Bottom line

SHBG is the liver's adjustable gate on sex hormone availability. It binds testosterone more tightly than estradiol, so changes in it shift the hormonal balance rather than simply diluting it. High SHBG can leave a man symptomatic at a respectable total testosterone; low SHBG can flatter a free testosterone while flagging the insulin resistance that produced it. Order it alongside total testosterone and albumin so free testosterone can be calculated properly, read the four patterns rather than the isolated value, and treat what moved the SHBG rather than the SHBG itself. It moves over months, so re-test on that timescale. The 60-second assessment routes a full panel to a physician who reads it in context.

Educational content, not medical advice. Laboratory interpretation and any treatment decision are made by a licensed physician after individual evaluation. Individual results vary.