thyroid follicle

Each thyroid follicle is composed of a single layer of follicular cells (cuboidal to columnar depending on activity) surrounding a lumen filled with colloid, the storage form of thyroglobulin. Follicular cells trap iodide via the sodium-iodide symporter (NIS), which is then transported to the colloid where thyroid peroxidase (TPO) oxidizes iodide and iodinates tyrosine residues on thyroglobulin, forming MIT and DIT. Coupling of MIT/DIT forms T3 and T4, which remain stored in the colloid until TSH stimulates endocytosis of colloid, proteolysis of thyroglobulin, and release of T3/T4 into the bloodstream. Parafollicular (C) cells, located between follicles or within the follicular basement membrane, secrete calcitonin and are of neural crest origin, distinct from the follicular cells which are of endodermal origin.

Clinically, follicular architecture is crucial in diagnosing thyroid pathology: Graves disease shows hyperplastic, tall columnar follicular cells with scalloped colloid due to TSH receptor antibody stimulation; Hashimoto thyroiditis shows follicular atrophy with lymphocytic infiltration and Hurthle cell change; follicular adenoma and follicular carcinoma arise from follicular cells and are distinguished by capsular/vascular invasion (seen only in carcinoma). Papillary thyroid carcinoma, arising from follicular cells, shows characteristic nuclear features (Orphan Annie eyes, psammoma bodies) rather than follicular architecture alone. Medullary thyroid carcinoma arises from parafollicular C cells and is associated with MEN 2A/2B syndromes, secreting calcitonin as a tumor marker.