SummaryOBJECTIVE The objective was to evaluate the effect of131I treatment for hyperthyroidism on calcitonin secretion by thyroid C‐cells.DESIGN Determination of basal calcitonin levels and calcitonin secretory reserve before and aiter131I administration.PATIENTS Seventeen hyperthyroid patients (15 female, two male) were studied before, and 2 months after131I treatment, and 12 of these patients were restudied 8 months after131I treatment.MEASUREMENTS Calcitonin response was assessed by measuring basal and post calcium infusion calcitonin levels. Basal TSH, T3, and T4 levels were also determined at each study.RESULTS The rise of plasma calcium resulted in statistically significant increase of plasma calcitonin levels before131I treatment (10.9·2.4 pmol/l), while this response was significantly diminished 2 and 8 months after treatment (2.6·0.7 and 1.6·0.3 pmol/l, respectively). No correlation was found between the calcitonin response and age or plasma TSH.CONCLUSION Our results demonstrate that131I treatment for hyperthyroidism may seriously damage thyroid C‐cells and cause calcitonin defi