Interscapular brown adipose tissue (IBAT) activity is controlled by the 
sympathetic nervous system, and factors that influence thermogenesis appear to 
act centrally to modify the sympathetic outflow to IBAT. Cold exposure produces 
a rise in IBAT temperature as a result of the increase in sympathetic outflow to 
IBAT. This is associated with an increased thyroid activity. 
3,5,3'-triiodothyronine (T3) and T4 levels increase during strenuous exercise, 
and, at the end of the exercise bout, a decrease of T3 and T4 levels, with an 
increase in TSH during the following 4-5 days, is seen. We evaluated the effect 
of strenuous exercise on 5'-deiodinase (5'-D) activity in IBAT in normal 
environmental conditions and after short (30 min) cold exposure. 5'-D activity 
is lower in rats at basal condition. Short cold exposure (SCE) increases 5'-D in 
IBAT both in exercising rats and in sedentary rats. However, this increase is 
lower in exercising animals. Strenuous exercise can reduce 5'-D activity in 
normal environmental conditions and after SCE. Probably, other compensatory 
mechanisms of heat production are active in exercising rodents.