Exercise regulation of triiodothyronine metabolism

Negative caloric balance reduces triiodothyronine (T3) production in both humans and rodents. The effects of chronic voluntary exercise and various levels of caloric intake and balance on T3 metabolism were studied in adult male C57/BL6 mice to determine if exercise had any direct effects on T3 production in vivo and in vitro. Chronic voluntary exercise was induced by the addition of running wheels to cages for 28 days. Ad libitum-fed exercising mice ingested 20% greater calories (P less than 0.02), maintained body weight, and increased T3 production (53.1 +/- 5.3 vs. 42.3 +/- 3.4 ng.h-1.100 mg body wt-1; P less than 0.01). Exercising animals pair fed to sedentary ad libitum-fed controls decreased their body mass to an equivalent degree as underfed sedentary animals (caloric intake 75% of ad libitum-fed controls) but had increased T3 clearance compared with weight-matched underfed sedentary control (P less than 0.05). Animals that were underfed and exercised decreased their body weight to a greater extent (P less than 0.01) compared with the sedentary underfed group, but T3 production rates were equal. Activity of liver 5'-deiodinase activity was decreased almost 50% (P less than 0.01) during both exercise plus pair feeding and exercise plus caloric restriction but decreased only 28% during caloric restriction alone (P less than 0.01). T3 metabolic clearance and production rates in vivo were correlated to caloric intake (r = 0.73, P less than 0.01), but an interaction between exercise and caloric balance was observed. Chronic voluntary exercise modulates T3 metabolism via several mechanisms. Exercise has an apparent stimulatory effect independent of caloric intake, but also there are regulatory effects dependent on the absolute level of caloric intake and relative caloric balance.(ABSTRACT TRUNCATED AT 250 WORDS)