Hypothermia in iron deficiency due to altered triiodothyronine metabolism

Abstract

Iron-deficient rats become hypothermic and have an excessive catecholamine response when exposed to an ambient temperature of 4 degrees C. This is not due to changes in body insulation, since thickness is unaltered, since differences persist after removal of hair, and since cutaneous vasoconstriction is intact. On the other hand, oxygen consumption of iron-deficient animals at 4 degrees C is reduced, 39 +/- 3 ml . kg-1 . min-1 compared to 63 +/- 2 in control animals. Thyroxine (T4) values at 4 degrees C were 4.34 +/- 0.20 microgram/dl sera as compared to control values of 3.6 +/- 0.32. Triiodothyronine (T3) values of iron-deficient animals in the cold were 48 +/- 6.8 ng/dl as compared to 72 +/- 5.6 in control animals. Treatment of iron-deficient animals with iron was shown to normalize the plasma T3 response at 4 degrees C within 6 days. Thyroidectomized iron-deficient animals injected with T3 did not show hypothermia at 4 degrees C, whereas thyroidectomized iron-deficient animals injected with T4 showed hypothermia, increased catecholamines, and decreased T3 levels as compared to non-iron-deficient animals similarly treated. It is proposed that iron deficiency impairs conversion of T4 to T3 and that this is primarily responsible for the hypothermia observed.

Effects of administration of iron, iodine and simultaneous iron-plus-iodine on the thyroid hormone profile in iron-deficient adolescent Iranian girls

“…The study of Smith and co-workers (1994) has confirmed findings on iron deficiency, where increased sympathetic nervous system activity (Dillmann et al, 1979;Groeneveld et al, 1985) was coupled with overt hypothyroidism (Beard et al, 1989). On the other hand, iron deficiency may significantly reduce circulating levels of T4-5 0 deiodinase in rats (Beard et al, 1989), the enzyme responsible for the conversion of T4-T3, resulting in diminished conversion of T4-T3 (Dillman et al, 1980). Therefore, the aim of present study was to ascertain the effects of the administration of iron, iodine and iron þ iodine on the thyroid hormone profile in iron-deficient girls.…”

“…Recently Hess and co-workers (2002) showed that thyroid peroxidase activity is significantly reduced in iron deficiency anaemia. Rats with iron deficiency and moderate iron deficiency anaemia have reduced conversion of T4-T3 (Dillman et al, 1980), and lower serum T3 and T4 concentrations compared to controls (Tang et al, 1988). On the other hand, iron-deficient rats have significantly lower hepatic 5 0 -deiodinase activity, with hepatic production of T3 reduced to only 46% of that in controls (Beard et al, 1990b) reduced turn over of serum T3 and blunted the TSH response to exogenous TRH (Beard et al, 1989).…”

“…Few studies in humans have been carried out on this subject. Results of those studies show that in adults, iron deficiency is accompanied by reduced serum T4 and T3 as compared to healthy controls (Dillman et al, 1980;Martinez-Torres et al, 1984;Brigham and Beard, 1995). In addition, Zimmermann et al (2000a) and showed that iron supplementation improved the efficacy of iodized salt in goitrous children with iron deficiency.…”

“…Studies in animals and humans have shown that iron deficiency with or without anaemia impairs thyroid hormone metabolism. Nutritional iron deficiency has been shown to significantly lower the circulating levels of both thyroxine and triiodothyronine in rats (Chen et al, 1983;Beard et al, 1984;Brigham and Beard, 1995), and to reduce conversion of T4-T3 (Dillman et al, 1980). Iron deficiency also impairs thyroid metabolism in human studies.…”

“…Martinez-Torres and co-workers (1984) reported 10% lower T3 levels in human subjects with moderate to severe iron deficiency anaemia and iron deficiency without anaemia when compared to iron-replete subjects, and Beard and his co-workers showed that in irondeficient subjects, serum T3 and T4 levels significantly decreased (1990). The two initial steps of thyroid hormone synthesis are catalysed by thyroperoxidase, a haem-containing enzyme (Dillman et al, 1980;Martinez-Torres et al, 1984;Beard et al, 1990aBeard et al, , b, 1998. Severe iron deficiency may lower thyroperoxidase activity and interfere with the synthesis of thyroid hormones (Hurrell, 1997).…”