Thyroid Hormone Function & Synthesis : Mnemonics

Thyroid Hormone Functions

  1. Basal metabolic rate
  2. Blood sugar (increases glycogenolysis & gluconeogenesis)
  3. Break down lipids (lipolysis)
  4. Bone growth (synergism with growth hormone)
  5. Beta-adrenergic effects
  6. Brain maturation
  7. Babies – stimulation of surfactant production

Thyroid Hormone Synthesis

Mnemonic: ATE ICE

1. Active transport of iodide:

  • Circulation to thyroid follicular cells: via NIS (Na-Iodide Symporter)
  • Thyroid follicular cells to lumen: via Pendrin

2. Thyroglobulin (rich in Tyrosine): formed in follicular ribosomes

3. Exocytosis of thyroglobulin into follicular lumen: stored as colloid

4. Iodination: Peroxidase enzyme –

  • Oxidation of Iodide (I-) to Iodine (I2)
  • I2 + Tyrosine (Organification) = MIT (Monoiodotyrosine), DIT (Diiodotyrosine)

5. Coupling:

  • MIT + DIT = T3
  • DIT + DIT = T4

6. Endocytosis: of T3 and T4 containing colloid into follicular cells and liberation of free T3 and T4 after proteolysis (mediated by proteases). At the peripheries, T4 is de-iodinated into more active T3.

Another mnemonic: IPOCP
1. Iodide transport
2. Peroxidation
3. Organification
4. Coupling
5. Peripheral conversion

thyroid hormone synthesis
By Mikael Häggström, used with permission., CC0, via Wikimedia Commons
Potency4 times more than T4Less
Rate of secretionLess10 times more than T3
Onset of actionRapid (2-3 hours)Slow (2-3 days)
Half-life1 day1 week
StorageLessMore (high affinity to proteins)
Binding0.2% in unbound0.02% in unbound
Source20-25% by gland
75-80% by peripheral conversion
100% by gland

Inhibition of Thyroid Synthesis and Antithyroid Drugs

Metabolic stepInhibitor
Iodine transportInorganic anions (Perchlorate, Thiocyanate)
IodinationThionamides (PTU, Methimazole, Carbimazole), Iodides via Wolff-Chaikoff effect (Lugol’s iodone, KI)
Colloid resorptionColchicine, Li+, I-
Deiodination of DIT + MITDinitrotyrosine
Hormone releaseIodine, Iodides of Na and K
Peripheral conversion of T4 to T3Dexamethasone, Beta-blockers (Propranolol), Radiocontrast dye, PTU
Destruction of thyroid tissueRadioactive iodine

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