Brown adipose tissue thermogenesis in T3-treated rats.

T4 treatment results in an inactivation of brown adipose tissue (BAT) which has been attributed to a reduced need of thermoregulatory heat production. Since T3 formation in brown adipocytes is governed by a type II T4 5'-deiodinase which is inhibited by T4, we analyzed the possibility that results obtained by T4 treatment were due to a lack of T3 in the tissue. Hyperthyroidism was induced in adult rats by administration of T3 (50 micrograms/kg body weight daily s.c.). Euthyroid and hyperthyroid rats were maintained at 23 degrees C or exposed at 6 degrees C for 3 weeks. Hyperthyroid rats at 23 degrees C showed an increase in BAT mass and in DNA and total lipids contents; however, BAT thermogenic activity was depressed. BAT from cold-exposed hyperthyroid rats showed the same mass and DNA content than at 23 degrees C, but it showed an increase in thermogenic activity, this increase being lower than in cold-exposed euthyroid rats. We conclude that high levels of T3 in BAT do not stimulate the thermogenic activity of the tissue. On the contrary, they inhibit it in response to lower requirements of facultative thermogenesis, both at 23 degrees C and at 6 degrees C.