Severe Agranulocytosis and Thyroid Storm Triggered by Reinitiating Low-Dose Thiamazole: A Cautionary Case.

Antithyroid drug (ATD)-induced agranulocytosis is a rare but potentially life-threatening adverse effect. We report the case of a 51-year-old man who developed agranulocytosis after reinitiating treatment with low-dose thiamazole, which was complicated by right buccal cellulitis, leading to thyroid storm. He was diagnosed with Graves disease 4 years earlier and treated with thiamazole; however, he discontinued the treatment on his own after 1 year. Due to the recurrence of Graves disease, 5 mg of thiamazole was reinitiated. Thirty-five days later, thyroid storm occurred, owing to right buccal cellulitis with thiamazole-induced agranulocytosis. Thiamazole can induce agranulocytosis, even at low doses and when reinitiated, regardless of prior tolerance. This case emphasizes that prior tolerance to thiamazole does not preclude the possibility of life-threatening adverse events upon reinitiation, even at minimal doses. Even in the absence of adverse effects during the initial course of ATD therapy, physicians should remain vigilant for ATD-related adverse effects when reinitiating treatment, even at low doses, particularly after a prolonged discontinuation period. Given the severity of agranulocytosis, patient education at the time of prescription is crucial for the early recognition of symptoms, such as fever or sore throat, which may enable timely diagnosis and appropriate intervention.