Introduction: Melasma is a common acquired hypermelanosis seen mainly in women which occurs on the sun-exposed skin of the face. The exact cause of melasma is unknown. The majority of the cases are attributed to pregnancy, combined oral contraceptive pills, genetic factors, sun exposure, use of cosmetic products, thyroid disorders and anti-convulsant drugs. This study was performed to evaluate the relationship between thyroid autoimmunity and melasma. Methods: In this case-control study, 45 females with melasma who were 20-50 years old and were referred to our clinic and 45 age-matched healthy females with no history of melasma as the control group were evaluated. The skin and thyroid of all of them were examined carefully and their sera were collected and evaluated for thyroid function tests and anti thyroid peroxidase (anti TPO). Data was also statistically analyzed. Results: Serum anti TPO level was higher than normal in %24.4 of the patients and %6.7 of the controls. The difference between two groups was statistically significant (P=0.019). Serum T3 level was higher than normal in %75.6 of the patients and %48.9 of the controls. The difference between the two groups was also statistically significant (P=0.008). Mean serum anti TPO, T3 and TSH were higher in the case group than the control group, but the difference was not statistically significant. Conclusion: A possible relationship might be observed between thyroid autoimmunity and melasma.