Calcitonin

Calcitonin is a hormone produced by the parafollicular or 'C-cells' of the thyroid gland; it has a minor role in calcium regulation, but its importance in testing is quite different. Its main clinical use is as a tumour marker for medullary thyroid cancer — an uncommon type of thyroid cancer that arises specifically from these C-cells and overproduces calcitonin. Measuring blood calcitonin therefore helps in three ways. First, it aids diagnosis: a clearly raised level in someone with a thyroid nodule raises the suspicion of medullary thyroid cancer. Second, once such a cancer is treated (usually by surgery), calcitonin is followed over time as a sensitive way to detect any recurrence — a level that stays low is reassuring, while a rising level signals the cancer may have returned. Third, it is used in screening families with the inherited form of the disease (part of the MEN2 syndromes), where relatives carrying the gene may be checked. Calcitonin can also be mildly raised for non-cancer reasons (such as kidney impairment or certain medications), so a modestly high result is interpreted carefully and usually rechecked, not assumed to mean cancer.

• Medullary thyroid cancer — the key concern; a clearly raised level, especially with a thyroid nodule, points toward this cancer.
• A rising level after treatment — suggests the cancer may have recurred; used for monitoring.
• Inherited (MEN2) forms — relevant when screening gene-carrying family members.
• Non-cancer causes of a mild rise — kidney impairment, certain medications (e.g. proton-pump inhibitors), smoking — so a modest elevation is interpreted with care.

• A low or normal level — the expected, reassuring result.
• A persistently low level after treatment of medullary thyroid cancer — reassuring, suggesting no detectable recurrence.