Thyroid Hormone Transport and Actions

Thyroid hormones (TH) are essential for normal development, differentiation growth and metabolism of every cell in the body. The pro-hormone thyroxine (T4) is synthesized by the thyroid follicles together with a small amount of the biologically active hormone triiodothyronine (T3), which derives mainly from tissue T4 deiodination. Approximately 0.03% of total T4 and 0.3% of total T3 in serum are circulating in a free or unbound form while the major part of TH is bound to circulating plasma proteins. These plasma proteins are responsible for the maintenance of the large extrathyroidal pool of TH, but their function is otherwise not quite clear, since wide differences in their concentrations do not influence the thyroid functional status of the individual to any large degree [1, 2].