Calcitonin: Definition

Calcitonin is a hormone produced by specialized cells in the thyroid gland called C cells (also known as parafollicular cells). Its primary role in the body is to help regulate blood calcium levels — when calcium levels rise too high, calcitonin lowers them by slowing bone breakdown and reducing the release of calcium into the bloodstream. In pathology, calcitonin is important as a diagnostic marker: because C cells normally produce calcitonin, tumors that arise from C cells — particularly medullary thyroid carcinoma — continue to produce this hormone, and detecting calcitonin in tissue or measuring it in the blood is a key part of diagnosing and monitoring this cancer.

Why do pathologists test for calcitonin?

Calcitonin testing in pathology serves two main purposes:

To confirm the diagnosis of medullary thyroid carcinoma (MTC). MTC is a cancer that arises from C cells. Because C cells are the only thyroid cells that produce calcitonin, demonstrating calcitonin expression in a tumor is strong evidence that the tumor is of C-cell origin. This is particularly useful when the microscopic appearance alone is uncertain.
To identify C-cell hyperplasia. In some cases, C cells multiply abnormally without forming a cancer — a condition called C-cell hyperplasia. This can be an early sign of hereditary MTC or an associated finding alongside an established MTC. Calcitonin staining helps identify and quantify these hyperplastic C cells.

How is calcitonin tested in tissue?

Pathologists detect calcitonin in tissue using a technique called immunohistochemistry (IHC). An antibody designed to bind specifically to the calcitonin protein is applied to a thin slice of thyroid or tumor tissue on a glass slide. Where calcitonin is present, the antibody triggers a color change — typically brown — visible under the microscope. The staining appears in the cytoplasm of C cells.

Normal C cells are scattered throughout the thyroid in small numbers and stain positively for calcitonin. In medullary thyroid carcinoma, many more cells stain positively, often intensely and across large areas of the tissue sample.

How results are reported

Calcitonin IHC results are reported as either positive or negative:

Positive (reactive) — calcitonin protein is detected in the tumor cells. This supports a diagnosis of medullary thyroid carcinoma or confirms C-cell origin. In the context of a thyroid nodule or tumor, a positive calcitonin result is a diagnostically important finding.
Negative (non-reactive) — calcitonin protein is not detected. This finding helps exclude MTC and supports other diagnoses, such as papillary thyroid carcinoma or follicular thyroid carcinoma, which arise from different thyroid cell types and do not produce calcitonin.

Is calcitonin also measured in the blood?

Yes. In addition to its use as a tissue marker, calcitonin is one of the few hormones used as a blood-based tumor marker. Because MTC cells produce and secrete calcitonin into the bloodstream, measuring calcitonin levels in a blood sample can:

Aid in diagnosis — a significantly elevated serum calcitonin level in a patient with a thyroid nodule raises a strong suspicion for MTC before a biopsy is performed.
Monitor treatment response — after surgical removal of MTC, calcitonin levels should fall. A drop to normal or undetectable levels suggests successful removal of the tumor.
Detect recurrence — rising calcitonin levels after treatment can be an early indicator that the cancer has returned, sometimes before it is visible on imaging.

What is the connection between calcitonin and inherited cancer syndromes?

Approximately 25% of medullary thyroid carcinomas are hereditary, caused by an inherited mutation in the RET gene. These hereditary forms are associated with a condition called multiple endocrine neoplasia type 2 (MEN2), in which affected individuals are at risk of developing MTC, certain adrenal tumors (pheochromocytoma), and other endocrine conditions. Because calcitonin is the hallmark marker of MTC, calcitonin testing — both in tissue and in blood — plays a central role in the screening and surveillance of individuals and families known to carry a RET mutation.

For more information about MTC and its hereditary forms, see the medullary thyroid carcinoma diagnosis guide.

Questions to ask your doctor

Was calcitonin testing performed on my tissue sample, and what did the result show?
What is my serum calcitonin level, and what does it mean for my diagnosis or monitoring?
Does my calcitonin result suggest any inherited cancer risk that my family members should know about?