Thyroid hemiagenesis is considered to be a rare congenital anomaly, but its incidence is propabley underestimated because the diagnosis is usually incidental. The left lobe is absent four times as often as the right and women are about three times more likely to have this developmental defect. Many of these patients are hyperthyroid or medical attention. We present the case of a 14-year-old women with right thyroid hemiagenesis associated without other thyroid disorders. The diagnosis of hemiagenesis was established by isotope imaging and thyroid ultrasound. In most of the clinical reports on thyroid hemiagenesis, an association with other thyroid disorders was found such as hyperthyroidism, multinodular goitre, hypothyroidism, benign adenama, Graves' disease, acute and subacute thyroiditis, and carcinoma. Even if morphology and function of thyroid gland is normal on diagnosis, a follow-up survey should be recommended.