The effect of radioactive iodine treatment on thyroid C cells

To determine the effect of radioiodine treatment on thyroid C cells, calcitonin (CT) levels were measured by RIA before and after intravenous calcium stimulation (2 mg/kg body-weight elemental calcium) in 22 women treated with 131I for hyperthyroidism. The results were compared with sex, age and weight-matched normal controls. There was a slight but statistically significant decrease in basal CT levels of the patients compared to the control group (mean +/- SE; 0.009 +/- 0.001 vs 0.011 +/- 0.001 pmol/l, P less than 0.05). The mean stimulated CT level of the patient group was significantly lower than that of the controls (0.010 +/- 0.001 vs. 0.018 +/- 0.003 pmol/l, P less than 0.001). The absence or presence of 131I-induced hypothyroidism at the time of the study did not influence basal or stimulated CT levels. Basal and stimulated CT levels were significantly lower in the patients with Graves' disease than in the patients with toxic nodular goitre. We conclude that 131I used to correct hyperthyroidism may cause marked CT deficiency.