Anti-thyroid drug therapy is considered a treatment of choice for Graves' disease; however, treatment response varies among individuals. Although several studies have reported risk factors for relapse after initial treatment, few have assessed responsiveness during the early treatment period. Our study aimed to identify the clinical characteristics for responsiveness to methimazole.

We included 99 patients diagnosed with Graves' disease for the first time. Drug responsiveness was defined as the correlation coefficients between decreasing rates of free thyroxine level per month and methimazole exposure dose. According to their responsiveness to treatment, the patients were classified into rapid or slow responder groups, and age, sex, free thyroxine level, and thyrotropin binding inhibiting immunoglobulin (TBII) titers were compared between groups.

The mean patient age was 44.0±13.5 years and 40 patients were male (40%). The mean TBII titer was 36.6±74.4 IU/L, and the mean free thyroxine concentration was 48.9±21.9 pmol/L. The rapid responder group showed higher TBII titer and free thyroxine level at diagnosis, while age, sex, smoking, and presence of goiter did not differ between the two groups. Logistic regression analyses revealed that high level of serum thyroxine, high titer of TBII, and absence of goiter were significantly associated with a rapid response, while age, sex, and smoking were not significant factors for the prediction of responsiveness.

In patients with new onset Graves' disease, high level of free thyroxine, high titer of TBII, and absence of goiter were associated with rapid responsiveness to methimazole treatment.