Change in the intrathyroidal kinetics of radioiodine under continued and discontinued antithyroid medication in Graves' disease.

PURPOSE: This study evaluated the thyroidal kinetics of radioiodine in Graves' disease under continued thiamazole medication and after discontinuation of thiamazole for 1-2 days, with a view to keeping the period of discontinuation as short as possible and to exploring the underlying mechanism of a postulated radioprotective effect of antithyroid drugs.
METHODS: In 316 patients, diagnostic and therapeutic radioiodine kinetics were followed up for 2 days by ten uptake measurements each and were defined mathematically by a two-compartment model.
RESULTS: Without thiamazole or when thiamazole was discontinued for at least 2 days, all uptake curves could be fitted perfectly by a simple in- and output function; the mean square error (mse) was 0.38 (test) and 0.28 (therapy). Under continued thiamazole medication (11.0+/-7.0 mg/day), the energy dose delivered to the thyroid was lowered by factor of 2.5. Uptake curves were deformed (mse: 1.06, test and 0.86, therapy) and appeared two peaked, suggesting coexistence of follicles with blocked and follicles with intact hormone synthesis and hence heterogeneous radioiodine uptake in the thyroid. In patients with maximally altered uptake curves, the success rate was as low as 31%. One day after discontinuation of thiamazole, mse was still increased (0.78, test), while 2 days afterwards it had normalised (0.36, test) and 3 days afterwards (mse: 0.24, therapy) the success rate was 87%.
CONCLUSION: Efficacy of radioiodine therapy under continued thiamazole medication is reduced not only by a lower uptake and shorter half-life of radioiodine, but also by a heterogeneous energy dose distribution in the thyroid. Discontinuation of thiamazole (but probably not of propylthiouracil) for at least 2 days is required to restore the efficacy of radioiodine.