Expansion
Iodide trapping, oxidation, organification, coupling, storage and release
Expansion
- Iodide trapping by the sodium-iodide symporter, concentrating iodide 30-fold. Blocked by perchlorate and thiocyanate
- Oxidation of iodide to iodine by thyroid peroxidase
- Organification: iodination of tyrosine residues on thyroglobulin, giving monoiodotyrosine and diiodotyrosine
- Coupling: two diiodotyrosines give T4; one mono plus one di gives T3
- Storage in colloid, sufficient for 2 to 3 months
- Release by endocytosis and proteolysis
Output is about 80 per cent T4 and 20 per cent T3, but T3 is the active hormone, mostly generated by peripheral deiodination of T4 in liver, kidney and muscle.
Pharmacological points:
- Carbimazole and propylthiouracil inhibit thyroid peroxidase; propylthiouracil also blocks peripheral conversion, which is why it is preferred in thyroid storm
- Because of the large stored pool, these drugs take several weeks to work
- High-dose iodide transiently inhibits synthesis and release, the Wolff-Chaikoff effect, used briefly before thyroid surgery