thyroid follicle

If the thyroid is a factory, the follicle is a single workshop with its own walled-off storeroom. Each follicle is a tiny sphere: a ring of cells surrounding a central pool of gel-like material called colloid, where finished and half-finished hormone is kept in reserve.

The wall of the follicle is made of follicular cells that face the colloid on their inner surface and the bloodstream on their outer surface. They pump iodide in from the blood, secrete the protein thyroglobulin into the colloid, and there assemble thyroid hormones on that protein. When the body needs hormone, the cells take colloid back in and release thyroxine and triiodothyronine into the circulation.

This follicular design is unusual in endocrinology because it lets the thyroid store weeks of hormone supply outside the cells, in the colloid. That reserve is why a person can be deprived of dietary iodine for some time before hormone levels actually fall.