SHBG is the variable that explains why two men with the same total testosterone feel completely different. One has plenty of free T and feels fine. The other has SHBG that is locking most of his testosterone away, and he feels like his levels are crashing. The total number on the lab is identical.
This is a field note on the most under-discussed marker in the hormone panel.
What is SHBG?
Sex hormone binding globulin is a protein made primarily by the liver. It circulates in the bloodstream and binds tightly to testosterone, dihydrotestosterone, and estradiol. Once a hormone is bound to SHBG, it cannot cross cell membranes and cannot bind androgen receptors. It is, for biological purposes, sidelined.
About 60 to 70% of testosterone in a healthy adult man is bound to SHBG. Another 30 to 40% is loosely bound to albumin. Only about 2% is genuinely free and biologically active.
Move SHBG up, and the bound fraction grows. The free fraction shrinks. Symptoms follow.
Why does SHBG matter for symptoms?
The biology only cares about the free fraction. Total testosterone is bookkeeping. What your androgen receptors actually see is the free testosterone plus the loosely bound albumin fraction, which together make up "bioavailable" testosterone.
A man with high SHBG can have a total T of 700 ng/dL and a free T placing him in the bottom 10% for his age. He will report low libido, soft erections, fatigue, mood changes, and trouble holding lean mass. His lab will say his testosterone is "normal" or even "high normal." He is, biologically, hypogonadal.
What drives SHBG up?
The most common causes in the men we see:
- Age. SHBG rises slowly through adulthood, accelerating after 40.
- Alcohol. Daily or near-daily drinking elevates SHBG meaningfully.
- Hyperthyroidism. A hot thyroid drives SHBG up sharply.
- Liver dysfunction. The liver makes SHBG; chronic liver stress raises it.
- Caloric restriction. Sustained underfeeding raises SHBG within weeks.
- Some medications. Certain anticonvulsants, oral estrogens, and a handful of others.
- Low insulin states. Counterintuitively, lean men with very low insulin can run high SHBG.
The pattern that catches us most often: a 42-year-old man who eats clean, runs a low-carb diet, drinks three glasses of wine a night, and works out hard. His total T looks fine. His SHBG is 70 nmol/L. His free T is in the basement.
What drives SHBG down?
The other direction:
- Insulin resistance. Even moderate insulin elevation suppresses SHBG.
- Obesity, particularly visceral. Closely related to the insulin signal.
- Hypothyroidism. A slow thyroid lowers SHBG.
- Androgens (testosterone, oxandrolone). Exogenous androgens push SHBG down, which can make free T disproportionately higher than total T predicts.
- Growth hormone elevation. Pharmacologic or endogenous.
A man with metabolic syndrome often has low SHBG. His total T may look concerning, but his free T is preserved because more of what he has is unbound. The labs say low T. The biology says fix the metabolic problem.
What is a normal SHBG level?
Reference ranges differ by lab, but typical adult male ranges fall between roughly 10 and 57 nmol/L.
What the number means:
- Below 20 nmol/L. Often low. Look for insulin resistance, obesity, or hypothyroidism.
- 20 to 45 nmol/L. Middle range. Most men sit here.
- Above 50 nmol/L. Often high. Look for alcohol, age, hyperthyroidism, or undereating.
Numbers are not absolutes. SHBG is one input. The clinically meaningful question is what it does to the free T calculation.
How SHBG changes the free T number
A simple sketch. Two men, both with total T of 600 ng/dL.
- Man A: SHBG 25 nmol/L. Calculated free T about 13 ng/dL. Normal, asymptomatic.
- Man B: SHBG 75 nmol/L. Calculated free T about 7 ng/dL. Symptomatic, low.
Same headline number. Different biology. SHBG is the lever.
How do you lower SHBG?
The answer is upstream, not pharmacologic. If SHBG is high because of alcohol, reduce it. If it is high because of hyperthyroidism, treat the thyroid. If it is high because of sustained caloric restriction, eat more for a period and see what moves.
Exogenous androgens lower SHBG, but starting TRT specifically to lower SHBG is the wrong sequence. Fix the upstream driver first. If symptoms persist after SHBG comes down and free T is still low, then the conversation about therapy makes sense.
How do you raise SHBG?
If your SHBG is low because of insulin resistance, fixing the metabolic picture (training, sleep, weight loss if indicated, sometimes a GLP-1) raises SHBG and improves the overall hormonal picture.
This is one of the underappreciated reasons that metabolic health and hormonal health are not separable.
How often should you check SHBG?
For most men, once at baseline alongside total T, free T, and the rest of the panel. Recheck if you change your training, your alcohol intake, your diet, or your weight meaningfully, or if you start a hormonal therapy.
We never make a treatment decision off a hormone panel that does not include SHBG. The single number tells us whether to believe the total T or to look past it.
Where Vane lands
SHBG is the variable that turns "low T" from a binary diagnosis into a biological story. Read it together with total T, free T, and the upstream signals, and the picture becomes clear. Read it in isolation, and you will misdiagnose half the men in this category.
If you have ever been told your testosterone is fine but you still feel like it is not, the next lab to check is SHBG. The number underneath the number is usually where the answer lives.
