Summary
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 μg/kg body weight daily s. c.). Euthyroid and hyperthyroid rats were maintained
at 23 °C or exposed at 6 °C for 3 weeks. Hyperthyroid rats at 23 °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 °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 °C and at 6 °C.
Key words
Brown Adipose Tissue - T3 Treatment - Cold-Exposure
