What is the major carrier protein of thyroid hormones in the blood?

• A. Albumin
• B. Thyroxine-binding globulin
• C. Thyroxine-binding prealbumin
• D. Thyroglobulin

Answer: (B)

Thyroxine-binding globulin binds the majority of circulating thyroid hormones in the blood. Thyroid hormones are lipophilic and need transport proteins to circulate; thyroxine-binding globulin, produced by the liver, has high affinity for T4 and T3 and acts to extend their half-life and maintain a stable reservoir of hormone. Albumin and thyroxine-binding prealbumin also carry thyroid hormones but to a lesser extent, providing additional capacity and buffering, while thyroglobulin is the protein stored in the thyroid for hormone synthesis and is not the circulating carrier. The active fraction is the unbound (free) hormone, with the bound portion serving mainly as a reservoir.