Background: Until now, it remains elusive which strategy - antithyroid drug withdrawal or increased radioiodine target doses - should be preferred to avoid the detrimental effect of antithyroid drugs in high- and low-dose radioiodine therapy, respectively.
Methods: We explored the effects of carbimazole on the 1-year post-radioiodine success and hypothyroidism rates by continuous dose-effect models, whereas success was defined as elimination of hyperthyroidism. Euthyroidism rates with and without carbimazole were calculated by numerical integration of the area between success and hypothyroidism curves. Target dose amplification factors for equal chance of success with and without carbimazole were calculated using logistic regression.
Results: Two hundred and twenty-eight patients were included in this study. Radioiodine target doses between 33 and 839 Gy were applied. Overall, the euthyroidism rates were 16.5% and 64.8%, while the hypothyroidism rates were 37.6% and 14.8% in Graves' disease and toxic nodular goitre, respectively. The success rate with simultaneous carbimazole (median dose 15 mg day(-1); range 2.5-60 mg day(-1)) was reduced over the entire target dose range in Graves' disease and toxic nodular goitre. The areas between curves for euthyroidism without and with simultaneous carbimazole were 127 and 43 Gy in Graves' disease and 178 and 128 Gy in toxic nodular goitre. The estimated radioiodine target dose amplification factor was 5.5 for Graves' disease and 3.0 for toxic nodular goitre.
Conclusions: Simultaneous carbimazole reduces the euthyroidism rate, the aim of low-dose radioiodine therapy, over the entire target dose range in both Graves' disease and toxic nodular goitre. Therefore, antithyroid drug discontinuation should be preferred in low-dose radioiodine therapy. Conversely, escalation of the target dose should be preferred in high-dose radioiodine therapy.