Thyroxine-binding globulin (TBG) is a globulin that binds thyroid hormones in circulation. It is one of three transport proteins (along with transthyretin and serum albumin) responsible for carrying the thyroid hormones thyroxine (T4) and triiodothyronine (T3) in the bloodstream. Of these three proteins, TBG has the highest affinity for T4 and T3 but is present in the lowest concentration. Despite its low concentration, TBG carries the majority of T4 in the blood plasma.

This test measures thyroglobulin antibodies commonly present in patients with Hashimoto's thyroiditis. Antibodies against the protein thyroglobulin can result in the destruction of thyroid cells. This destruction can lead to hypothyroidism. NOTE: Dietary supplements containing biotin may interfere in assays and may skew results to be either falsely high or falsely low. For patients receiving the recommended daily doses of biotin, draw samples at least 8 hours following the last biotin supplementation. For patients on mega doses of biotin supplements, draw samples at least 72 hours following the last biotin supplementation.

This panel includes comprehensive testing of the thyroid stimulating hormone (TSH) in addition to free thyroxine (T4) and free triiodothyronine (T3).

The thyroid gland produces and secretes T4, otherwise known as thyroxine. Proteins bind to T4 and carry it throughout the bloodstream. Once in the tissues, T4 is released from the proteins and is now free to convert into the more active form called T3. For this reason, many doctors believe that measuring free T4 is a more sensitive test for thyroid hormone production.