Thyroid Hormone Secretion: Clarifying Calcitonin's Cellular Origin

Calcitonin is not secreted by extrafollicular cells. It is produced exclusively by the parafollicular cells (C cells) of the thyroid gland.

Cellular Context:

• Follicular cells: Form the thyroid follicles; synthesize and secrete thyroxine (T4) and triiodothyronine (T3) under TSH stimulation.
• Parafollicular (C) cells: Located singly or in small clusters in the basal lamina of follicles and in the interstitium; derived from neural crest (ultimobranchial body); secrete calcitonin in response to hypercalcemia.
• "Extrafollicular cells" is not a standard thyroid histological term. It may colloquially refer to C cells (which are outside follicles) or stromal/immune cells. Only C cells produce calcitonin.

Physiological Role: Calcitonin lowers blood calcium by inhibiting osteoclast-mediated bone resorption and increasing renal calcium excretion. It opposes parathyroid hormone (PTH). Its role in human calcium homeostasis is minor compared to PTH and vitamin D; clinical relevance is primarily as a tumor marker for medullary thyroid carcinoma (C-cell neoplasm).