Box 3.29Metabolism of thyroid hormones

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  • Thyroid hormones are metabolized by a series of deiodinations which involve three types of deiodinases (indicated by numbers in brackets)
    • Type 1: deiodinates at both the 5′ and 5 carbon atoms and is found in the liver, kidney, thyroid, pituitary gland and central nervous system. With a high Km for T4, it is the only isoenzyme inhibited by PTU. Its activity is increased in hyperthyroidism and reduced in hypothyroidism.
    • Type 2: deiodinates only at the 5′ position and is found in brain, brown fat, placenta and pituitary gland. With a lower Km than Type 1, it is considered to maintain intracellular concentrations of T3. This is important in the negative feedback actions of T4 on the pituitary gland. Its activity is decreased in hyperthyroidism and increased in hypothyroidism.
    • Type 3: deiodinates only at the 5 position and is found only in brain and placenta. As it is incapable of converting T4 to the active T3, it may protect the brain and fetus from excess active T3.
  • Some T4 is metabolised by being sulfated, decarboxylated, deaminated or conjugated with glucuronide (other pathways).
  • Some T3 may be sulfated (T3S) or converted to the acetic acid derivative triiodoacetic acid (TRIAC) that is more potent than its parent T3.
  • Serum half lives: T4 — 7 days, T3 — 1 day, rT3 — 4 hours.

From: Chapter 3, The thyroid gland

Cover of Endocrinology
Endocrinology: An Integrated Approach.
Nussey S, Whitehead S.
Copyright © 2001, BIOS Scientific Publishers Limited.

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