Clinical Studies on Potassium Iodide-induced Painless Thyroiditis in 11 Graves' Disease Patients

OBJECTIVE: Painless thyroiditis (PT) is characterized by transient hyperthyroidism with a low (99m)Tc uptake. We herein describe 11 cases of PT that occurred during treatment with potassium iodide (KI) for Graves' disease (GD). METHODS: From August 2016 to December 2018, 11 women with GD who developed PT during treatment with KI were enrolled. Of these patients, 10 discontinued antithyroid drug (ATD) because of side effects and began KI, and 1 patient switched from thiamazole to KI because she was planning a pregnancy. The mean patient age was 40.1 years old. Thyroid function tests, thyroid autoantibodies including anti thyroglobulin antibody (TgAb), anti-thyroperoxidase antibody (TPOAb), and M22-TRAb, and the (99m)Tc uptake were evaluated at the time of PT. RESULTS: All 11 women patients presented with transient thyrotoxicosis in which (99m)Tc scans revealed a low uptake of 0.34±0.15% (normal 0.70-1.02%). M22-TRAb was absent in all cases except for one (2.4 IU/L), whereas TgAb and TPOAb were present in 10 and 6 cases, respectively. Ten patients returned to a euthyroid status without passing through the post-hypothyroid phase, and one patient underwent total thyroidectomy during the euthyroid phase of PT. Only four patients require beta-blocker therapy. All patients with KI-induced PT except 1 displayed GD remission during a mean observation period of 23.3 months, and 1 patient had recurrence of GD after PT. CONCLUSION: We encountered 11 GD patients who developed PT during treatment with KI, which was initiated after ATD had been discontinued due to side effects.