Thyrocyte

Thyrocyte, also called a follicular cell, is the epithelial cell that lines the follicles of the thyroid gland. These cells are derived from endoderm and form a simple cuboidal to low columnar epithelium surrounding a lumen filled with colloid rich in thyroglobulin.

Primary functions include the synthesis and secretion of thyroid hormones. Thyrocytes produce thyroglobulin, a large glycoprotein that serves as the scaffold for hormone synthesis. They express thyroid peroxidase and thyroglobulin, and they actively take up iodide from the blood via the sodium-iodide symporter. Iodide is organified by thyroid peroxidase and incorporated into tyrosine residues of thyroglobulin to form MIT and DIT, which couple to produce the hormones T3 and T4. Thyrocytes store hormone within the colloid as thyroglobulin and release it into the bloodstream after endocytosis and proteolysis of thyroglobulin following stimulation by TSH.

Regulation is primarily through pituitary TSH and hypothalamic TRH. Thyrocyte activity and thyroid hormone production are controlled by a negative feedback loop in which circulating T3 and T4 suppress TSH and TRH release.

Histology and anatomy: thyrocytes are polarized cells with the basal membrane in contact with capillaries and the apical surface facing the colloid; tight junctions help maintain distinct luminal and systemic environments.

Clinical relevance: thyrocyte dysfunction can cause hypothyroidism or hyperthyroidism and may be part of conditions such as goiter, Hashimoto’s thyroiditis, or Graves’ disease. The term emphasizes the functional epithelial cell of the thyroid, distinct from parafollicular C cells that secrete calcitonin.