Poor compliance or drug malabsorption are the most common reasons why an adequate TSH suppression is not achieved with oral levothyroxin in patients with hypothyroidism or thyroid carcinoma. When these conditions are excluded rare causes have to be considered. We report a female patient with follicular thyroid carcinoma in whom, under intended levothyroxin suppression therapy, a TSH-PRL-producing pituitary adenoma manifested by failure to achieve adequate TSH suppression, subtle signs of hyperthyroidism,and finally symptoms of elevated PRL.