Null Mutant Mice for Thyroid Hormone Receptors

In mammals, thyroid hormones (TH) have been shown to control the postnatal development of many organs, such as brain, intestine and long bone, and to participate in the maintenance of homeostasis in adults by controlling basal metabolism, heart rate, and body temperature (1 ,2 ). To ensure this last role, circulating TH concentrations are maintained very stable by a tight control of TH production. Indeed, TH, which is primarily synthesized in the thyroid gland, represses the production of two peptidic hormones, thyrotropin-releasing hormone (TRH) in the hypothalamus and thyroid-stimulating hormone (TSH) in the pituitary. TRH normally stimulates the production of TSH, which, in turn, stimulates the thyroid gland and thus permits an efficient TH production (3 ).