Thyroid hemiagenesis (TH), very rare congenital anomaly, is generally asymptomatic. We report two cases of TH with hyperthyroidism. Case One. The patient presented with signs and symptoms of thyrotoxicosis. Physical examination revealed asymmetric nodular goitre at right lobe. Biochemical analysis revealed the diagnosis of hyperthyroidism. Ultrasound showed multinodular hypertrophy in the right lobe and absence of the left lobe. Nuclear scan, confirming absence of the left lobe, showed hot nodules in the right one. The diagnosis was toxic multinodular goitre. Case Two. The thyroid was not palpable in this patient presented with signs and symptoms of thyrotoxicosis. Biochemical analysis revealed the diagnosis of autoimmune thyrotoxicosis. Ultrasound showed mild diffuse hyperplasia of the right lobe and agenesis of the left lobe. Nuclear scan, confirming absence of the left lobe, showed increasing diffuse uptake of radiotracer in the right one. The diagnosis was Graves' disease in this patient. After antithyroid medication, the patients were surgically treated with total excision of the thyroid tissue. TH is sometimes associated with disorders of the thyroid. Hyperthyroidism makes TH cases symptomatic. During evaluation of patients, ultrasound and nuclear scan usually report agenesis of one lobe and establish the diagnosis of TH. The surgical treatment is total removal of hyperactive tissue and total excision of the remaining lobe.