THERMOGENESIS AND THYROID-FUNCTION

The past 10 years have seen tremendous progress in the definition of t he nuclear mechanism of action of thyroid hormones. Although the way i n which these nuclear mechanisms underlie the 3,5,3'-triiodo-L-thyroni ne (T-3)-dependent stimulation of metabolic rate remains to be clarifi ed, evidence favoring nonnuclear pathways is limited. Clearly, T-3 sti mulates both the production and consumption of energy within cells, It also exerts a number of parallel effects that result in increased oxy gen consumption, e.g. on mitochondrial structure and composition; on t he metabolism of lipids, carbohydrates, and proteins, and on cardiac f unction. Additionally, T-3 may increase the proton permeability of the inner mitochondrial membrane, which implies that it may decrease the efficiency of energy production. These metabolic effects of T-3 appear to be restricted to homeothermic animals, representing a coordinated response to the challenge of maintaining body temperature.