Serum thyroid hormone-binding proteins

Thyroid hormones circulate in the blood mostly and reversibly bound to a set of thyroid hormone binding proteins, the most important being thyroxine-binding globulin, transthyretin and albumin. These proteins may undergo genetic and inherited variations, either decrease or excess. These variations affect measurement of serum total thyroid hormones, but do not influence the levels of free (unbound to proteins) thyroid hormones, which represent the minute, but metabolically active fraction at the tissue level.