The calcitonin test is primarily ordered to help diagnose C-cell hyperplasia and medullary thyroid cancer (MTC), to evaluate the effectiveness of treatment for MTC, and to monitor patients for recurrence. It is also used to screen for MTC in family members of patients with MEN 2.

Stimulation tests are more sensitive than calcitonin measurements alone. This involves collecting a baseline sample, then giving the patient an injection of intravenous calcium or pentagastrin to stimulate calcitonin production. Several more blood samples are then collected over the next few minutes to gauge the effect of the stimulation. Patients with early C-cell hyperplasia and/or MTC will usually have very significant increases in their levels of calcitonin during this test.

Calcitonin levels may be ordered when a patient is suspected of having C-cell hyperplasia or medullary thyroid cancer. A recent study also recommended measuring calcitonin before surgery in all patients with thyroid nodules, but not all authorities agree. Stimulation tests may be ordered when a patient has indeterminate or normal calcitonin results but clinical suspicions remain. These tests may be regularly performed on family members of patients who have MEN 2, starting at an early age, in order to detect MTC or C-cell hyperplasia as soon as it occurs. When someone has been treated for MTC, calcitonin testing will usually be ordered at regular intervals to monitor treatment effectiveness and recurrence.

An elevated concentration of calcitonin means that excessive amounts are being produced. Significantly elevated levels of calcitonin are a good indicator of C-cell hyperplasia or MTC; however, the doctor will use other procedures, such as a thyroid biopsy, scan, and ultrasound, to confirm the diagnosis.

With successful treatment for MTC (removal of the thyroid gland and often some surrounding tissues), calcitonin levels will usually fall to very low levels. If the values stay low over time, then it is likely that the treatment was effective. In some cases, calcitonin levels will fall but remain moderately elevated after treatment. This means that some calcitonin-producing tissue remains. Doctors will monitor calcitonin and watch for increases over time. If calcitonin levels begin to rise, then it is likely that there is a recurrence of MTC.

Other thyroid tests, such as T4, T3, and TSH, are usually normal when calcitonin levels are elevated.