Effect of thyroid hormone replacement in iron-deficient rats.

To determine if the previously observed alterations in norepinephrine (NE) metabolism and resting metabolic rate in iron-deficient (ID) rats result from hypothyroidism, exogenous thyroxine (T4) and 3,5,3'-triiodothyronine (T3) were administered to ID rats in doses sufficient to normalize the plasma concentrations of these hormones, whereas other ID and control (CN) rats received placebo treatment. Resting oxygen consumption was approximately 25% higher in ID than CN rats; T4 but not T3 treatment alleviated this elevated oxygen uptake. The NE content of interscapular brown adipose tissue (IBAT), liver, and heart was 70-80% lower in ID than CN rats, and NE turnover in the same tissues was likewise 40-60% lower in ID than CN rats, with no systematic effect of either T3 or T4 treatment. Liver T(4)5'-deiodinase activity was 70% lower in ID than CN rats and increased with T4 but not T3 treatment. These experiments show that iron deficiency alters NE and energy metabolism in a way that is mostly independent of its effects on thyroid hormone metabolism.