The first case of idiopathic thyrotropin (TSH) deficiency in an old woman with thyroid functioning adenoma was reported. She got subtotal thyroidectomy before about four years of her admission to our hospital because of fatigability, puffy face and leg edema. At time, she had low TSH and free T4 levels despite replacement therapy with desiccated thyroid. No response of only serum TSH after administration of combined stimulant containing TRH and repeated TRH suggested the failure of TSH secretion. CT MRI did not show any abnormality. These results indicated that her hypothyroidism was due to acquired idiopathic TSH deficiency.