Sex hormone-binding globulin

Sex hormone-binding globulin (SHBG) is a blood plasma protein that binds testosterone, dihydrotestosterone, and estradiol, influencing how much hormone circulates unbound.

Free hormone hypothesis: The hypothesis that unbound steroid hormone is the fraction most available to enter cells and exert biological effects. It explains why SHBG-bound hormone is distinguished from the unbound fraction.
Free testosterone: The fraction of circulating testosterone not bound to SHBG or albumin. Its concentration is often estimated or measured when total testosterone alone may mislead.
Steroid hormone: A hormone built from a cholesterol-derived structure that can cross cell membranes and bind intracellular receptors. Testosterone and estradiol are steroid hormones carried in blood partly by SHBG.
Thyroid hormone: Hormones produced by the thyroid gland that regulate metabolism, growth, and development. Hyperthyroidism can raise SHBG, while hypothyroidism can lower it.
Testosterone: A steroid sex hormone involved in reproductive development, sexual function, muscle and bone physiology. Testosterone is a major SHBG ligand, and its bound fraction depends on SHBG concentration.
Bioavailable testosterone: Circulating testosterone that is unbound or weakly bound to albumin and considered available to tissues. It includes albumin-bound hormone as well as free testosterone, unlike the free fraction alone.
Plasma protein: A protein dissolved in blood plasma, where it can transport molecules or support physiological functions. SHBG is a specialized plasma protein rather than a hormone or receptor.
Estrogen: A group of steroid hormones involved in reproductive development and other physiological processes. Estrogen exposure can increase hepatic SHBG production.
Dihydrotestosterone: A potent androgen formed from testosterone by the enzyme 5α-reductase. SHBG binds dihydrotestosterone more strongly than testosterone.
Total testosterone: A blood-test measure of testosterone that includes free and protein-bound hormone. SHBG can shift total testosterone without a matching change in free testosterone.

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