Sex Hormone Binding Globulin

Most TRT clinicians target 20–40 nmol/L. Too high (>50) and free T gets squeezed; too low (<15) and you may need to cut total testosterone to avoid symptoms of androgen excess.

Address insulin resistance (lose visceral fat, improve fasting glucose), treat hypothyroidism if present, reduce alcohol, and ensure adequate protein intake. Some clinicians also use boron supplementation though evidence is mixed.

Lose body fat, address fatty liver, increase fiber, and ensure thyroid function is normal. Some men also benefit from splitting their TRT into more frequent, smaller injections — which paradoxically can raise SHBG over time.

SHBG is a protein made mostly by the liver that binds testosterone and estradiol in the bloodstream and carries them in an inactive, storage form. Because bound hormone can't enter cells, SHBG effectively regulates how much of your testosterone is biologically available. It's the biggest determinant of free testosterone after total T itself.

The adult male reference range is roughly 16.5–55.9 nmol/L, though it varies by lab and assay. SHBG is reported in nmol/L on standard panels. Levels tend to rise gradually with age — about 1% per year after 40.

The most common drivers are obesity, insulin resistance, fatty liver, and metabolic syndrome — high insulin suppresses liver SHBG production. Hypothyroidism, Cushing's syndrome, and anabolic steroid use also lower it, and some men simply have genetically low SHBG. Low SHBG means a larger share of your testosterone is free.

Aging is the biggest factor, along with hyperthyroidism, liver disease, and chronic calorie restriction or anorexia. Oral estrogens and some anti-androgens raise it too, and a subset of men are just genetically high. High SHBG lowers free testosterone even when total T looks normal.

SHBG binds testosterone tightly, while albumin binds it loosely; the small fraction bound to neither is free testosterone (~1–3% of total). At the same total T, higher SHBG leaves less free T, and lower SHBG leaves more. This is why two men with identical total testosterone can feel completely different — their SHBG differs. Calculated free T formulas like Vermeulen use total T, SHBG, and albumin together for this reason.

SHBG carries sex hormones (testosterone and estradiol), while other transport proteins each carry different hormones — thyroxine-binding globulin carries thyroid hormone and corticosteroid-binding globulin carries cortisol. They're separate proteins with separate reference ranges, so a thyroid or cortisol panel won't tell you anything about your SHBG. On a hormone panel, SHBG is the one relevant to testosterone.