Thyroid epithelial cells, also known as follicular cells, are cells in the thyroid gland which produce thyroxine (T4) and triiodothyronine (T3). The follicular cells use iodine from the blood to make thyroid hormone, which helps regulate a person's metabolism. The main function of the thyroid gland is to take iodine, found in many foods, and convert it into thyroid hormones: thyroxine (T4) and triiodothyronine (T3).
Thyroid epithelial cells are the only cells in the body which can absorb iodine. These cells combine iodine and the amino acid tyrosine to make T3 and T4. T3 and T4 are then released into the blood stream and are transported throughout the body where they control metabolism (which is the conversion of oxygen and calories to energy). Every cell in the body depends upon thyroid hormones for regulation of their metabolism. The normal thyroid gland produces about 80% T4 and about 20% T3, however, T3 is about four times as potent as T4. They are simple cuboidal epithelium and are arranged in spherical follicles surrounding colloid.
They have thyrotropin receptors on their surface, which respond to thyroid-stimulating hormone. Embryologic origin is from a median endodermal mass in the region of the tongue (foramen cecum) in contrast to the parafollicular (C) cells that arise from the 4th branchial pouch.