thyroxine-binding globulin

Thyroxine-binding globulin is the chauffeur for thyroid hormone in the blood. Thyroid hormones do not dissolve well in plasma and would be lost too quickly on their own, so most ride bound to this carrier protein, escorted safely to the tissues.

Made by the liver, it is the main of several plasma proteins that bind thyroxine and triiodothyronine. The vast majority of circulating hormone is bound and inactive; only the tiny free fraction can enter cells and act. The carrier therefore acts as a buffer and reservoir, smoothing supply and protecting hormone from being cleared too fast, while keeping the active free level steady.

This carrier matters for interpreting lab tests. Conditions that raise the carrier — pregnancy or estrogen therapy — increase total hormone in the blood without making the person hyperthyroid, because the free level stays normal. That is exactly why clinicians prefer to measure free hormone and TSH: total-hormone measurements can be misleading whenever carrier levels shift.