Calcitonin is a hormone produced by the parafollicular cells, also known as C-cells, located within the thyroid gland. This small protein circulates in the bloodstream, helping maintain a stable internal environment. Its primary function is related to balancing calcium and phosphate within the body. When levels are found to be abnormal, it often prompts a medical investigation to determine the underlying cause.
How Calcitonin Regulates Calcium
Calcitonin regulates blood calcium concentration, working in opposition to parathyroid hormone (PTH). The thyroid’s C-cells release calcitonin in response to increased circulating calcium. This action is part of a feedback loop designed to prevent calcium levels from rising too high.
The hormone primarily acts on bone tissue to achieve its calcium-lowering effect. It works by inhibiting osteoclasts, specialized cells responsible for breaking down bone tissue (bone resorption). When osteoclasts are inhibited, the release of stored calcium and phosphate from the bone matrix into the bloodstream is reduced.
Calcitonin also has a secondary effect on the kidneys. It decreases the reabsorption of calcium and phosphate back into the blood by the renal tubules. This promotes the excretion of these minerals through the urine. However, the exact importance of calcitonin in daily calcium regulation in healthy adults is not fully understood, as PTH appears to be more dominant.
Measuring Calcitonin Levels
A calcitonin test measures the hormone’s concentration in the serum or plasma via a simple blood draw. It is often ordered when a person has a thyroid nodule or a family history of Medullary Thyroid Carcinoma (MTC). Calcitonin serves as a tumor marker, indicating the presence or recurrence of this specific cancer.
Calcitonin concentration naturally varies; men generally have higher basal levels than women. Reference ranges differ between laboratories, but basal levels are commonly considered normal if they are less than 19 pg/mL for men and less than 14 pg/mL for women.
In certain diagnostic situations, a more sensitive stimulation test may be performed. This involves drawing a baseline sample, then injecting a substance (such as calcium or pentagastrin) to provoke C-cells to release calcitonin. Multiple samples are collected to measure the peak response, revealing subtle C-cell abnormalities.
Interpreting Abnormal Results
The most common and clinically significant finding is elevated calcitonin, known as hypercalcitoninemia. An increased concentration is highly suggestive of Medullary Thyroid Carcinoma (MTC), a rare cancer originating from the thyroid’s C-cells. Levels above 100 pg/mL are highly suspicious, and the degree of elevation often correlates with the tumor burden and cancer stage.
Calcitonin testing is essential for monitoring patients treated for MTC. Successful surgery typically results in a sharp decrease in the calcitonin level, often to an undetectable amount. Conversely, a rising or persistently high level after treatment indicates that cancerous tissue remains, or that the cancer has returned or spread.
Hypercalcitoninemia can also be caused by C-cell hyperplasia, the abnormal, non-cancerous growth of C-cells. C-cell hyperplasia is often considered a precursor to MTC, especially in inherited forms, and may produce moderately elevated calcitonin levels. Distinguishing between hyperplasia and MTC often requires stimulation tests and genetic analysis.
Other non-thyroid conditions can also cause high calcitonin levels, complicating interpretation. These include chronic kidney failure, certain non-thyroid neuroendocrine tumors, and conditions causing high stomach acid (hypergastrinemia). Medications like proton pump inhibitors, and physiological states such as pregnancy, may also lead to transient elevations.
In contrast, a low calcitonin level is generally not considered a cause for medical concern in a person with an otherwise healthy thyroid. Low or undetectable levels are an expected finding following a total thyroidectomy, as the primary source of the hormone has been removed. The absence of the hormone does not negatively affect the body’s ability to maintain normal blood calcium levels, as parathyroid hormone and other regulatory mechanisms compensate effectively.