Creatine Kinase Elevation During Antithyroid Treatment of a Patient with Graves’ Disease: A Case Report and Review of Literature

Abstract Thionamides (methimazole and propylthiouracil), which have been used in the treatment of graves’ disease since 1940, inhibit the organification of iodine and coupling of iodotyrosines, thus blocking the synthesis of hormones. Myalgia is a rare side effect of these drugs. CK is the muscle specific kinase. The measurement of serum concentration of CK is useful to estimate the muscles’ breakdown. We present a young male patient with Graves’ disease who had abnormal increase of CK level during treatment with methimazole (MMI). He experienced myalgia and elevated CK level 1 month after initiation of MMI. In the beginning of the myalgia, his free T4 level decreased to normal range. After dose reduction of MMI, CK level decreased and his symptoms were resolved. Although the mechanisms for this effect are not yet clear, it is thought that rapid decrease in thyroid hormone by antithyroid treatment in susceptible patients with graves’ disease can be the cause of CK elevation. Measuring CK level in Graves disease patients presented with myalgia during treatment with antithyroid drugs would be a useful diagnostic tool.