Calcitonin, Serum

Calcitonin, Serum (thyrocalcitonin, CT) is used to aid diagnosis, stage disease, and monitor treatment response in medullary thyroid carcinoma. Persistently elevated or rising basal values after thyroidectomy help assess residual or recurrent MTC. The assay also assists in identifying multiple endocrine neoplasia type 2A (MEN2A/Sipple syndrome) in appropriate clinical contexts, such as in patients with MTC, pheochromocytoma, or primary hyperparathyroidism. As an adjunct, basal calcitonin may be incorporated into the evaluation of calcium–phosphate disorders, including hyperparathyroidism, hypoparathyroidism, and primary osteoporosis.

Thyroid C cells secrete calcitonin, which has a short plasma half-life of approximately 2–15 minutes. Secretion increases as serum calcium rises and decreases when serum calcium falls. Calcitonin acts on osteocyte–osteoclast systems to curb bone resorption and lower circulating calcium, serving as a physiologic counterbalance to parathyroid hormone; this regulatory axis is influenced by vitamin D3. In medullary thyroid carcinoma, basal calcitonin concentrations generally reflect tumor burden. Provocative testing with agents such as pentagastrin or intravenous calcium can reveal early C‑cell pathology: pronounced post‑stimulation elevations favor MTC, whereas modest or within‑reference increases may be consistent with early C‑cell hyperplasia. Following thyroidectomy for MTC, serial calcitonin measurements assist in determining biochemical cure and in surveillance for persistent or recurrent disease. Calcitonin secretion can also be modulated by nonthyroidal factors, including gastrointestinal hormones (eg, pepsin) and pancreatic glucagon. Pancreaticobiliary and hepatic conditions—such as pancreatitis, acute cholecystitis, and cirrhosis—may secondarily affect circulating calcitonin levels.