A case of autoimmune thyroiditis presenting as menstrual related mood disorder (MRMD) is described. The symptoms of the patient's prospectively confirmed MRMD remitted following thyroid hormone supplementation. Although most patients with prospectively confirmed MRMD are not clinically hypothyroid or hyperthyroid, the importance of routine thyroid function tests in the initial evaluation of MRMD is underscored by the successful treatment of this patient with thyroid hormone replacement.