Thyroid Hormone Metabolism

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Human Uniqueness Compared to "Great Apes": 
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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.


Timing of appearance of the difference in the Hominin Lineage as a defined date or a lineage separation event. The point in time associated with lineage separation events may change in the future as the scientific community agrees upon better time estimates. Lineage separation events are defined in 2017 as:

  • the Last Common Ancestor (LCA) of humans and old world monkeys was 25,000 - 30,000 thousand (25 - 30 million) years ago
  • the Last Common Ancestor (LCA) of humans and chimpanzees was 6,000 - 8,000 thousand (6 - 8 million) years ago
  • the emergence of the genus Homo was 2,000 thousand (2 million) years ago
  • the Last Common Ancestor (LCA) of humans and neanderthals was 500 thousand years ago
  • the common ancestor of modern humans was 100 - 300 thousand years ago

Definite Appearance: 
100 thousand years ago
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TTR (transthyretin)


  1. Proteomic comparison of human and great ape blood plasma reveals conserved glycosylation and differences in thyroid hormone metabolism., Gagneux, P, Amess B, Diaz S, Moore S, Patel T, Dillmann W, Parekh R, and Varki Ajit , Am J Phys Anthropol, 2001 Jun, Volume 115, Issue 2, p.99-109, (2001)