INHIBITION OF METABOLISM BY REVERSE T3 (3,3′,5′-TRIIODOTHYRONINE) IN THE LAMB

Publisher Summary It has been known for a number of years that thyroxine (T 4 ) is monodeiodinated to T 3 and rT 3 in the body. Synthetic exogenous rT 3 had been reported to be thermogenetically inactive and to inhibit the metabolic effects of T 4 . A number of studies have shown that its plasma concentration is affected by changing physiological conditions. This has led to the suggestion that rT 3 is a natural inhibitor of T 3 acting competitively at specific binding sites within the cell. This chapter presents the results of an investigation of the time course of the inhibitory effects of rT 3 on short-latency T 3 -stimulated metabolism. No changes in metabolic rate occurred in the 23 controls. With both the lower and higher dose of T 3 many, but by no means all, animals showed a clear elevation of metabolic rate. A careful inspection of individual results showed that T 3 treatment either caused a considerable metabolic stimulation or failed to cause any response at all. It was demonstrated that 10 μg/kg rT 3 had an inhibitory effect on metabolism and in some cases had the ability to reverse or arrest the trend of increasing VO 2 in T 3 pretreated animals.