Thyrocytes

Thyrocytes, also known as follicular cells, are the epithelial cells that line the thyroid follicles in the thyroid gland. They form a continuous layer around a colloid-filled lumen and are the primary functional component of the gland. The parafollicular C cells, which secrete calcitonin, are distinct from thyrocytes.

Thyrocytes synthesize thyroglobulin, a large glycoprotein that is secreted into the follicular lumen and serves as the scaffold for thyroid hormone synthesis. Iodide is transported from the bloodstream into thyrocytes via the sodium-iodide symporter on the basolateral membrane and then moves into the colloid via apical transporters such as pendrin. Thyroperoxidase catalyzes iodination and coupling reactions on thyroglobulin, producing the hormones T3 (triiodothyronine) and T4 (thyroxine), which remain attached to thyroglobulin within the colloid.

Upon stimulation, thyrocytes endocytose iodinated thyroglobulin, proteolyze it in lysosomes, and release free T3 and T4. The hormones then diffuse across the basolateral membrane into capillaries. T3 is the more active form, while T4 is converted to T3 in peripheral tissues. Synthesis and secretion are regulated primarily by thyroid-stimulating hormone (TSH) from the pituitary, with negative feedback by circulating T3 and T4. The Wolff-Chaikoff effect describes temporary suppression of synthesis in response to excess iodine.

Thyrocytes originate from endoderm and are central to controlling metabolic rate, development, and growth. Pathologies involving thyrocytes include autoimmune conditions such as Hashimoto’s thyroiditis, which damages thyrocytes and reduces hormone production, and Graves’ disease, where TSH receptor-stimulating antibodies cause thyrocyte hyperactivity.