Thyroid hemiagenesis is usually diagnosed incidentally during examination for other thyroid gland diseases. Iodine-123 thyroid scan of a 25-year-old woman showed a normal homogeneous right thyroid lobe, but no uptake in the left lobe. Ultrasonography of the neck confirmed the absence of the left thyroid lobe, while the right lobe was homogeneous and normal. Thyroid hemiagenesis should be included in the differential diagnosis of thyroid diseases.