Carbimazole-induced myositis in the treatment of Graves' disease: a complication in genetically susceptible individuals?

A 24-year-old Chinese woman with Graves' disease presented with myositis two months after treatment with carbimazole. The patient's myositis resolved with hydration and cessation of carbimazole. No other causes of myositis were found, and a change in the medication to propylthiouracil was uneventful. Review of the literature suggests a possible genetic susceptibility, as the majority of reported cases are Asian in origin, similar to patients who present with thyroid periodic paralysis. Changing the antithyroid drugs (ATDs) administered, decreasing the dose of pre-existing ATDs in the treatment regimen or addition of levothyroxine has been shown to result in clinical improvement of this complication. These observations suggest various mechanisms of carbimazole-induced myositis in the treatment of Graves' disease, including the direct effect of ATDs on myocytes, immune-related responses secondary to ATDs and rapid decrements in thyroid hormone with ensuing myositis.