In congenital thyroid hemiagenesis, one of the thyroid lobes, usually the left one, does not develop. This condition is generally asymptomatic. We report a 47 years old female with a diffuse left goiter. An ultrasound detected the right lobe hemiagenesis. She was subjected to a subtotal thyroidectomy. The pathological study of the surgical piece disclosed the presence of an encapsulated papillary carcinoma, follicular variant with a diameter of 2 mm.