We present a 5 years 8 months old girl with severe hyperlipidemia (high total cholesterol, and low density lipoprotein values, and also, ectopic fat pericardial deposit). She was treated with diet and cholestyramine, without diagnosis of her disease etiology. Growth detention, weight loss, retarded bone age and clinical signs of hypometabolism were recorded. Thyroid profile confirms hypothyroidism diagnosis. Based on positive anti-thyroid antibodies and clearly reduced thyroid volume, a diagnosis of autoimmune atrophic thyroiditis was made, a very unusual pathology in early infancy. Linear growth was affected by late diagnosis.