In mammals, iodine functions biochemically as a component of thyroid hormones, which are required for many aspects of normal metabolism, growth, and development. In addition to the iodine that is required as a central component of the thyroid hormones, selenium is essential for thyroid hormone synthesis and activation and for their proper metabolism. The role of selenium in biological systems is much more complex than previously supposed. Selenium deficiency may cause abnormal thyroid hormone metabolism1–3 that then results in decreased growth rate, reduced fertility, and impaired immune responses and resistance to infectious diseases.4 Thyroxine, which is the major hormone product of the thyroid gland, is biologically inactive and needs to be converted to its active metabolite T3 via 5a-monodeiodination.5,6 Deiodination of T4 to T3 is catalyzed by selenoenzymes, namely deiodinase types I, II, and III. These enzymes contain selenium as seleno cystein at their active sites.7–9 Another effect of selenium on thyroid hormone metabolism is derived from its action on selenium-containing GSH-Pxs and thioredoxin reductase, which protect the thyroid gland from H2O2 that is released during the synthesis of thyroid hormones.10–12
The discovery of the influence of selenium on thyroid hormone metabolism has concentrated considerable research into the effects of selenium deficiency on thyroid hormone homeostasis in humans and other species. Nutritional deficiency of selenium in rats results in decreases in the activity of deiodinase type I and serum T3 concentration.13 A similar situation has been observed in calves that are fed diets low in selenium.14,15 Effects of a low-selenium diet on serum concentrations of T4 and T3 in calves,1 heifer calves on pasture,15 Angus and crossbred beef cows,16 and housed dairy cows consuming preserved food17 have also been reported. However, there is limited information about thyroid hormone status in association with naturally occurring clinical forms of selenium deficiency in farm animals kept under field conditions. The purpose of the study reported here was to assess changes in serum concentrations of thyroid hormones associated with selenium deficiency myopathy in lambs. The lambs included in the study were affected with a subacute form of selenium deficiency myopathy (nutritional myodegeneration or so-called stiff-lamb disease), which is the most well-described and common clinical form of selenium deficiency in large animals.
Ultraspec Plus, UV visible 4054, LKB Pharmacia, Uppsala, Sweden.
Ransel, Randox Laboratories Ltd, Crumlin, Ireland.
CK kit, Pars Azmoun, Tehran, Iran.
Technicon RA 1000 system, Technicon Instruments Corp, New York, NY.
TSH enzyme immunoassay kit, Amersham Pharmacia Biotechnology, Piscataway, NJ.
Accurate Chemical & Scientific Corp, Westbury, NY.
Total T4 ELISA kit, Human Gesellschaft für Biochemica und Diagnostica GmbH, Wiesbaden, Germany.
Total T3 ELISA kit, Human Gesellschaft für Biochemica und Diagnostica GmbH, Wiesbaden, Germany.
SPSS software, version 9.0, SPSS Inc, Chicago, Ill.
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