Povidone-iodine-induced transient triiodothyronine thyrotoxicosis in a Japanese patient with prolonged habitual gargling: A case report and literature review

Rationale: Iodine-induced hyperthyroidism and triiodothyronine (T3) thyrotoxicosis in patients who routinely gargle with povidone-iodine (PVP-I) gargling solution are rare in Japan.Patient concerns: A 50-year-old man presented to our hospital for a close examination of an enlarged thyroid, which was noted during a complete health checkup. The thyroid was slightly enlarged with no palpable nodules. He had an increased appetite but no weight gain. He had been routinely gargling with PVP-I gargling solution 4 times daily for >10 years. He had no history of thyroid disease.Diagnoses: Test results revealed suppressed thyroid-stimulating hormone, normal free thyroxine, and increased free triiodothyronine levels, leading to the diagnosis of T3 thyrotoxicosis.Interventions: The patient agreed to stop gargling with PVP-I gargle solution.Outcomes: The free triiodothyronine and thyroid-stimulating hormone levels returned to normal at 18 and 21 weeks, respectively, after discontinuation of PVP-I gargling. After an improvement in thyroid function, he gained 5 kg in 1 year.Lessons: To our knowledge, this is the first case report that describes PVP-I gargle-induced T3 thyrotoxicosis in a healthy individual without thyroid disease. In Japan, which is an iodine-sufficient country, considering the possibility of high-dose iodine intake-induced thyrotoxicosis due to long-term PVP-I gargling or other causes is necessary, even in individuals with no history of thyroid disease.