Effect of 131 I treatment on the calcitonin response to calcium infusion in hyperthyroid patients

OBJECTIVE: The objective was to evaluate the effect of 131I treatment for hyperthyroidism on calcitonin secretion by thyroid C-cells. DESIGN: Determination of basal calcitonin levels and calcitonin secretory reserve before and after 131I administration. PATIENTS: Seventeen hyperthyroid patients (15 female, two male) were studied before, and 2 months after 131I treatment, and 12 of these patients were restudied 8 months after 131I 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 before 131I 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 that 131I treatment for hyperthyroidism may seriously damage thyroid C-cells and cause calcitonin deficiency.