Metabolic rate and thyroxine monodeiodinase activity in iron-deficient female Sprague-Dawley rats: Effects of the ovarian steroids

Abstract

Iron deficiency results in altered thyroid hormone metabolism throughout the estrous cycle. To assess the interactions of the ovarian steroids, thyroid hormones, and iron deficiency, we examined indices of thyroid function in ovariectomized iron-deficient animals and control animals treated with estradiol (mean plasma concentration, 260 pmol/L), progesterone (mean plasma concentration, 45 nmol/L), estradiol and progesterone, or no steroid (sham-treated). Neither steroid affected plasma triiodothyronine (T 3) concentrations relative to sham-treated animals, but estradiol treatment (either alone or with progesterone) elevated plasma thyroxine (T 4) concentrations ( P ⩽ 0.001). Plasma T 3 and T 4 concentrations were lower in iron-deficient ovariectomized rats ( T 3 = 0.38 ± 0.08 nmol/L; T 4 = 22.0 ± 9.7 nmol/L) relative to ovariectomized controls ( T 3 = 0.44 ± 0.11 nmol/L; T 4 = 27.4 ± 8.4 nmol/L; P ⩽ 0.05 and 0.01, respectively. Hepatic thyroxine monodeiodinase activity was lower in iron-deficient rats (422 ± 241 versus 565 ± 251 pmoles I · mg protein −1 · 20 minutes −1; P ⩽ 0.03). Brown adipose tissue deiodinase activity decreased in estradiol-treated groups relative to sham-treated animals ( P ⩽ 0.03); however, the brown adipose tissue thyroxine monodeiodinase was unaffected by iron deficiency. Metabolic rates were elevated in iron-deficient rats ( P ⩽ 0.003) and were lower in estradiol-treated rats. Iron-deficient rats exhibit an altered thyroid response to exogenous ovarian steroids. In conclusion, plasma parameters of thyroid function are altered in iron-deficient rats, and these appear to be affected by the ovarian steroids. Tissue deiodinase activities are lower and metabolic rates are higher in iron-deficient rats.