Determinations in 11 patients with anti-thyroid drug therapy of hyperthyreoidism showed, that a pronounced lag period exist between normalisation of thyroid hormone level and TRH responsiveness. The lag period range between 5 and 27 weeks. During this time it is not necessary to give simultaneously thyroid hormones in antithyroid drug therapy to compensate hyperresponsiveness of TSH. - Investigations in 12 volunteers with experimental hyperthyreoidism induced by administration of thyroxine suggest a longterm suppression of the hypophysis to be the cause of the lag period of TSH-secretion. There are some similarities to the ACTH-suppression in therapy with corticosteroids and to the so-called "syndrome of oversuppression" in long lasting treatment with contraceptive drugs.