iodide trapping

Iodide trapping is how the thyroid acts like a magnet for a scarce raw material. Iodide is rare in the diet, so the gland actively pulls it out of the blood and concentrates it many times over against its natural tendency to spread out.

The work is done by a pump in the follicular cell membrane called the sodium-iodide symporter, which rides the inward flow of sodium to drag iodide in alongside it. This is the very first and rate-limiting step of hormone synthesis: without trapped iodide there is nothing for thyroid peroxidase to attach to thyroglobulin. Thyroid-stimulating hormone increases the number and activity of these pumps.

This trapping mechanism has practical consequences. It lets doctors give radioactive iodine that the thyroid selectively absorbs, for imaging or for treating overactive glands and cancer. It also explains why potassium iodide tablets are stockpiled for nuclear emergencies: flooding the gland with ordinary iodide blocks uptake of dangerous radioactive iodine.