follicular cell

Follicular cells are the line workers of the thyroid factory. Each one sits in the follicle wall with one face toward the blood and the other toward the colloid, shuttling raw materials in one direction and finished hormone in the other.

From the blood side they trap iodide and pull in nutrients; into the colloid they secrete thyroglobulin and, with the help of thyroid peroxidase, build thyroxine and triiodothyronine on it. When stimulated, they reabsorb colloid, break down the thyroglobulin, and release the hormones into the bloodstream. All of this activity is switched up or down by thyroid-stimulating hormone acting on the TSH receptor at the cell’s blood-facing surface.

Follicular cells are also where most thyroid cancers arise (papillary and follicular types), and because they uniquely concentrate iodide, radioactive iodine can be used to image or selectively destroy them — a rare instance of treatment that homes in on a single cell type. The thyroid’s other endocrine cells, the calcitonin-producing C cells, are a separate lineage and do not behave this way.