Objectives: To evaluate the prevalence of hypergastrinemia in patients with hyperthyroidism and hypothyroidism and to determine whether gastrin-induced hypercalcitonemia could explain the high prevalence of thyroid C-cell hyperplasia among patients with hyperthyroidism. Methods: Concentrations of gastrin and of hCT were determined by commercially available radioimmunoassays. Results: Elevated serum concentrations of gastrin were found in 17 of 161 (10.5%) patients with manifest hyperthyroidism (Graves’ disease) and in 4 of 37 (10.8%) and 23 of 255 (9.0%) patients with manifest or subclinical hypothyroidism, respectively. Only 2 cases of hypergastrinemia of 255 subclinically hypothyroid patients (0.8%) could not be linked to thyroid autoimmune disease by either biochemical or sonographic criteria. Four patients with Graves’ disease presented elevated plasma concentrations of calcitonin, but none of these patients also had an elevated serum gastrin. Conclusions: The prevalence of hypergastrinemia in autoimmune thyroid disease is about 10%. The determination of gastrin in subclinical hypothyroidism is not cost-effective in the absence of biochemical and/or sonographic markers of autoimmune thyroid disease. The determination of gastrin is of no use to predict the presence of C-cell hyperplasia commonly seen in patients with Graves’ disease.