Calcitonin

Calcitonin is a polypeptide hormone secreted by the parafollicular (C cells) of the thyroid. Its main action is inhibition of bone resorption by regulating the number and activity of osteoclasts. Calcitonin is secreted in direct response to elevated plasma calcium levels. It prevents large fluctuations in plasma calcium levels and excessive loss of body calcium. However, compared to parathyroid hormone and 1,25 OH-vitamin D, calcitonin plays a minor role in plasma calcium regulation. Measurement of plasma calcitonin levels is not useful in the diagnosis of hyper- or hypocalcemia.  

Medullary thyroid carcinoma arises from thyroid C cells and usually produces elevated calcitonin levels. Medullary carcinoma is uncommon, comprising only 5% of thyroid malignancies. Approximately 25% of cases are familial, usually presenting as a component of multiple endocrine neoplasia type II (MEN II). Medullary thyroid cancer may also occur in families without MEN II and is then classified as familial medullary thyroid cancer. Both MEN II and familial medullary thyroid cancer are associated with mutations in the RET proto-oncogene.

Although most patients with sporadic medullary thyroid cancer have high plasma calcitonin levels at the time of diagnosis, 30% of patients with familial medullary thyroid cancer or MEN II have normal levels at the time of diagnosis. These cases may require molecular genetic testing for the RET proto-oncogene. 

Following successful surgical removal of medullary carcinoma, plasma calcitonin levels gradually decrease until they become undetectable over a period of several weeks. Persistently elevated levels usually indicate incomplete cure. A rise in a previously undetectable or very low postoperative calcitonin level is highly suggestive of tumor recurrence.   

Several other medical conditions may increase serum calcitonin levels including:

• Active autoimmune thyroiditis
• Hyperparathyroidism
• Renal failure
• Pernicious anemia
• Sepsis
• Massive trauma
• Pregnancy at term
• Lactation
• Mastocytosis
• Neonates
• Non-thyroid neuroendocrine tumors including islet cell, carcinoid and small cell carcinoma of the lung

Often times, these conditions only produce modest elevations (2-4 fold) above the upper limit of the reference range. 

Calcitonin can be measured with an automated chemiluminescent immunoassay. The adult reference range is 0-8 pg/mL in females and 0-14 pg/mL in males.

Specimen requirement is one plain red top tube of blood. Calcitonin is unstable and about 20% is degraded into fragments every 2 hours at room temperature or in a refrigerator. For this reason, serum should be frozen immediately.