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Article Type

Case Report

Abstract

Background Thyroid hormones regulate many physiologic functions such as energy and heat production, synthesis of proteins, and muscle contraction. Objective This study aims to demonstrate the mechanism by which hyperthyroidism affects neuromuscular performance and acute exercise tolerance and whether this effect is reversible or not after recovery in rats. Materials and methods Rats used in this study were divided into three groups, each containing 10 rats: (a) in the euthyroid group, rats were selected by measurement of serum FT3 (triiodothyronin), FT4 (tetraiodothyronine), and thyroid-stimulating hormone levels. (b) In the hyperthyroid group, hyperthyroidism was induced by Eltroxin in increasing doses, beginning from 50 μg to reach 200 μg/kg body weight daily by intragastric administration for 3 weeks. (c) In the recovery group, hyperthyroid rats are allowed to recover by exogenous thyroxine withdrawal for 25 days. After induction, body weight and the maximal swimming time were estimated, and then retro-orbital blood samples were collected for estimation of malondialdehyde and the total antioxidant capacity. Rats were then killed by cervical decapitation. Phrenic nerve diaphragm preparations were excised, connected to a four-channel oscillograph for measuring the strength of muscle contraction at the start of the experiment and after 30 min activity using both direct and indirect stimulation. Glucose uptake by the muscle during 30 min of activity and 30 min of recovery was estimated. Results The data show that hyperthyroidism induced by Eltroxin elevates serum malondialdehyde significantly when compared with the euthyroid group, whereas reduces total antioxidant capacity significantly. Hyperthyroidism was also associated with reduction of body weight, the maximal swimming time, the strength of skeletal muscle contraction induced by direct and indirect repetitive stimulation, and its glucose uptake. Conclusion Oxidative stress plays a critical role in the pathogenesis of hyperthyroidism, especially on skeletal muscle performance and exercise tolerance. This effect is reversible after restoration of the euthyroid state.

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