This is a prospective, randomized, double-blind, placebo-controlled study about the effect of metformin in patients with mild Graves' ophthalmopathy. Eighty patients with mild ophthalmopathy will be included. Each patient will be given either metformin (500mg PO three times a day) or placebo ( PO three times a day ) for 6 months and then followed for 6 months after withdrawal of treatment.

Inclusion Criteria:

1. Age 18-80 years
2. Patients should not have diabetes (HbA1c level at screening should be lower than 6.5)
3. Mild Graves' ophthalmopathy according to EUGOGO statement.
4. Clinical activity score lower than 4
5. Being euthyroid for at least 2 month before the enrollment
6. No previous specific therapy for GO in 6 month before the enrollment except for local measures (e.g. eye drops)

Exclusion Criteria:

1. Moderate-severe Graves' ophthalmopathy
2. Clinical activity score ≥ 4
3. Contraindication for metformin use as following: renal dysfunction (creatinine level should be within normal range), liver dysfunction (AST and ALT levels should be within normal range), severe concomitant illness
4. Pregnant women
5. Current use of metformin or containing preparations
6. Metformin intolerance
7. Inability to comply with the study protocol