Down syndrome (DS) is the most common chromosomal abnormality in humans, and the most frequent cause of mental retardation. Patients affected by this syndrome show an increased prevalence of autoimmune diseases. The most common of those is Hypothyroidism. We present a case report describing the association of Down syndrome with Hyperthyroidism. An 18-year-old patient presented with a history of recurrent throat infections and intermittent diarrhea, having developed a total alopecia areata within one month from the first visit to the physician. After consultations with general practitioners, he was directed to an Endocrinology Ambulatory and diagnosed with a clear case of Graves' disease associated with Down syndrome. Treatment was started with methimazole 20 mg/day, and after two months, was adjusted to 40 mg/day. The patient reached adequate clinical and laboratory balance after five months of treatment. Thus, the association between Down syndrome and Graves' Disease is relevant in medical practice, due to its specific characteristics on diagnosis, and the need of an adequate treatment regarding this disease association.