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Evidence for Thyroid Hormone Deficiency in Iron-Deficient Anemic Rats^1,2,

Nutrition Department, The Pennsylvania State University, University Park, PA 16802

Iron-deficient anemic rats have previously been shown to have low plasma levels of thyroid hormone and a poor plasma thyroid hormone response to acute cold exposure. As an initial exploration, we examined thyroid hormone metabolism during iron deficiency in age-matched rats from three aspects: 1) plasma TSH (thyrotropin, thyroid stimulating hormone), T₄ (thyroxine) and T₃ (triiodothyronine) responses to graded doses of exogenous TRH (thyrotropin releasing hormone), 2) plasma T₃ kinetics, and 3) rates of hepatic T₃ production. Iron-deficient anemic rats had lower basal TSH values and blunted TSH responses to intravenous TRH injection at three different doses (10, 25 and 50 ng TRH/100 g body wt). Iron-deficient anemic rats also had a significant decrease in plasma T₃ turnover (42 vs. 88 ng/h in controls), and significantly lower hepatic T₄-5'-deiodinase activities than controls [26 vs. 44.0 ng T₃/(mg protein · 20 min)]. Thus, decreased rates of T₃ production in iron-deficient anemic rats, as documented by turnover studies, may be related to decreased deiodinase activity and reduced peripheral formation of T₃. The dampened TSH responses to TRH further facilitate or perpetuate this T₃ deficiency. We propose that this abnormal thyroid state is partially responsible for impaired thermogenesis in iron-deficiency anemia.

KEY WORDS: • TRH • T₃ plasma kinetics • thyroxine 5'-deiodinase • thermoregulation • thermogenesis

¹ Supported in part by grants from the Medical Research Service, Veterans Administration, the Biomedical Research Support Grant Program and National Institutes of Health grant #DK39160.

² Part of this data were presented at the Federation of American Societies of Experimental Biology (FASEB) Annual Meeting, Las Vegas, NV, May 1988. FASEB J. 2(6): A1609 (abs. 7628).

³ Present address: Veterans Administration Medical Center, Brooklyn, NY.

Manuscript received 17 October 1988. Revision accepted 30 January 1989.