Graves' disease is the main cause of hyperthyroidism. Graves' disease has a prevalence of 0.5% in the general population. As a non-surgical treatment, antithyroid drug (ATD) and radioactive iodine treatment have been proposed and ATD is the first-line treatment in Korea. However, ATD has a rare but fatal side effect of agranulocytosis. Furthermore only half of its users maintain long-term remission and frequent recurrence is a problem to this disease. Therefore it is essential to distinguish between patients who respond well to ATD and those who resist it. The aim of this study is to verify the changes in gut microbiome in Graves' disease patients before and after six-month treatment with ATD. Patients first diagnosed with Graves' disease will participate in the study. The study design is a prospective longitudinal trial. The patients are asked to have their gut microbiome analyzed before and after the treatment of Graves' disease with ATD. Primary endpoint is the changes of analyzed gut microbiome before and after ATD treatment. Secondary outcome is to find species that can be used as a biomarker to differentiate the patients refractory to ATD.

Inclusion Criteria:

• More than 19 years and lesser than 70 years
• First diagnosed as Graves' disease and planned to have antithyroid drug

Exclusion Criteria:

• Pregnancy and lactating women
• Inflammatory bowel disease
• Active malignancy
• Autoimmune disease
• Immune-deficiency patients

Patients first diagnosed with Graves' disease, aged more than 19 years and lesser than 70 years. Eligible for treatment of graves' disease with antithyroid drug. Consented for collection of stools in 6-month interval