Graves disease is an autoimmune disorder of the thyroid gland. Patients with type 1 diabetes (T1DM) are at a higher risk of developing autoimmune diseases including thyroid disorders. To our knowledge, diagnosing Graves disease at the same time in a monozygotic twin with T1DM is a rare occurrence. We present a case of monozygotic female twin with T1DM, who both developed Graves disease at the same time. Type 1 diabetes was diagnosed in one of the twins (twin A) at the age of 10, 4 years later her sister developed type 1 diabetes at age of 14 years (twin B). They lived in the same house and went to same school and college. Both patients and their parents are nonsmokers. There has been no family history of autoimmune diseases or thyroid disorders. Four years before the diagnoses of Graves disease, Twin A was diagnosed to have autoimmune hypothyroidism while her sister (twin B) was noted to have a very high titre of TPO antibodies with evidence of subclinical hyperthyroidism which was managed conservatively. Treatment with thyroxine replacement was initiated for twin A lasting for 48 months, which was eventually terminated when both twins started to demonstrate clinical and biochemical features of hyperthyroidism at the same time. Graves orbitopathy was not present. Biochemical assays confirmed the diagnosis of Graves disease. Both patients had positive thyroid receptor antibodies (TRAb) which was negative 4 years earlier. Both patients were started on Carbimazole (CM) 15 mg daily. Following one year of treatment with CM, the 2 sisters were clinically and biochemically euthyroidal. This case report supports the hypothesis that genetic factors play an important role in the etiology of thyroid autoimmune diseases and may determine the time window of emergence of the thyroid autoimmunity.