PLASMA THYROID-HORMONE KINETICS ARE ALTERED IN IRON-DEFICIENT RATS

Iron deficiency anemia is associated with lower plasma thyroid hormone concentrations in rodents and, in some studies, in humans. The object ive of this project was to determine if plasma triiodothyronine (T-3) and thyroxine (T-4) kinetics were affected by iron deficiency. Studies were done at a near-thermoneutral temperature (30 degrees C), and a c ool environmental temperature (15 degrees C), to determine plasma T-3 and T-4 kinetics as a function of dietary iron intake and environmenta l need for the hormones. Weanling male Sprague-Dawley rats were fed ei ther a low Fe diet [iron-deficient group (ID), <5 mu g/g Fe] or a cont rol diet [control group (CN), 35 mu g/g Fe] at each temperature for 7 wk before the tracer kinetic studies. An additional ID group receiving exogenous thyroid hormone replacement was also used at the cooler tem perature, For T-4, the disposal rate was >60% lower (89 +/- 6 vs, 256 +/- 53 pmol/h, P < 0.001) in ID rats than in controls at 30 degrees C, and similar to 40% lower (192 +/- 27 vs. 372 +/- 26 pmol/h, P < 0.01) in ID rats at 15 degrees C. Exogenous T-4 replacement in a cohort of ID rats at 15 degrees C normalized the T-4 concentration and the dispo sal rate. For T-3, the disposal rate was significantly lower in ID rat s in a cool environment (92 +/- 11 vs, 129 +/- 11 pmol/h, P < 0.01); t hyroxine replacement again normalized the T-3 disposal rate (126 +/- 1 2 pmol/h). Neither liver nor brown fat thyroxine 5'-deiodinase activit ies were sufficiently different to explain the lower T-3 disposal rate s in iron deficiency. Thus, plasma thyroid hormone kinetics in iron de ficiency anemia are corrected by simply providing more thyroxine, This suggests a central regulatory defect as the primary lesion and not pe ripheral alterations.