Thyroid Hormone Metabolism

Thyroid hormones are powerful signal-generating molecules influencing development and metabolism of all vertebrates. Thyroid hormones are not very water soluble and, for this reason, have to be shuttled throughout the body to any target tissues by carrier molecules in plasma and cerebrospinal fluid (e.g. albumin, thyroxine-binding globulin and transthyretin). Thyroid hormones exist in two different forms (thyroxine = T4 and triiodothyronine = T3) each of which can be bound to a carrier protein or be free. T4 is a prohormone which gets converted into T3, the active form of the hormone. Measurements of thyroid hormone levels in plasma by radio-immunoassay have revealed higher concentrations of free T4, total T3, free T3 as well as T3 uptake (measuring unoccupied binding sites on binding proteins) in chimpanzees but higher total T4 in humans. Assuming that these findings are replicated, but they suggest a difference in thyroid hormone metabolism between humans and chimpanzees. These differences could be due, at least in part, to different concentrations of the transport protein transthyretin, in the plasma and cerebrospinal fluid of humans and chimpanzees. There are potential implications for differences in the development of the brain.