In patients with medullary thyroid carcinoma, calcitonin levels are often elevated. However, in a significant percentage of patients, calcitonin levels are normal or show only a borderline increase. When stimulated by an infusion of pentagastrin, calcium or both, patients with medullary thyroid carcinoma will show a marked increase in serum calcitonin levels. The combined stimulation test (pentagastrin plus calcium) is considered more reliable than either agent alone. The test can be used to screen family members of patients with the familial form of medullary thyroid carcinoma.