Rats fed low protein diets often exhibit an elevation in adaptive thermogenesis that may result in part from activation of thyroid hormone action at the level of peripheral deiodination of thyroxine (T4) to 3,5,3'-triiodothyronine (T3). This possibility was examined by comparing the kinetic parameters of iodothyronine 5'-deiodination in hepatic and renal microsomes and the efficiency of energy retention in weanling rats fed diets containing 5% (LP-5), 8% (LP-8) or 22% (NP) casein. Energy balance measurements revealed that efficiency of energy retention was reduced in rats fed LP-5, whereas no significant change in adaptive thermogenesis was observed in rats fed LP-8. In the LP-5 group the maximal activity of hepatic 5'-deiodinase per milligram of microsomal protein was not different from that of the NP group; however, the Vmax of the renal enzyme was 21% higher. In both tissues, the Km of the reaction was not affected by dietary treatment. Hepatic and renal 5'-deiodination in rats fed LP-8 was similar to that in rats fed LP-5. When compared to corresponding NP controls, each of the low protein dietary groups (LP-5, LP-8) had a higher serum total T3 concentration, whereas serum free T3 and total T4 concentrations were unaffected by the dietary treatments. The findings demonstrate that iodothyronine 5'-deiodination and circulating thyroid hormone concentrations vary in a similar manner in LP-5 and LP-8 animals even though these two groups exhibit different responses in energy efficiency relative to NP controls.(ABSTRACT TRUNCATED AT 250 WORDS)