Mechanisms of Thyroid Hormone Action on Basal Metabolic Rate and Exercise Tolerance: A Comprehensive Review
Keywords:
thyroid hormones, basal metabolic rate, exercise tolerance, hypothyroidism, hyperthyroidism, thermogenesis, mitochondrial uncouplingAbstract
Introduction: Thyroid hormones (TH), primarily triiodothyronine (T3) and thyroxine (T4), are fundamental regulators of metabolic homeostasis. Their influence on basal metabolic rate (BMR) and exercise tolerance represents a clinically significant interaction that affects millions of patients globally. This comprehensive literature review explores the physiological and molecular mechanisms through which TH regulate energy expenditure and physical performance capacity. Methodology: A systematic search of peer-reviewed literature published between 2014 and 2024 was conducted using electronic databases, including PubMed, Scopus, ResearchGate, and Web of Science. The search strategy focused on key terms such as "thyroid hormones," "basal metabolic rate," "exercise tolerance," "hypothyroidism," "hyperthyroidism," and "VO2 max." Findings: Evidence consistently demonstrates that hyperthyroidism is associated with a 25–80% increase in BMR; however, it paradoxically leads to decreased exercise tolerance due to cardiovascular overload, skeletal muscle thyrotoxic myopathy, and an increased risk of atrial fibrillation. Conversely, hypothyroidism reduces BMR by 15–40% and impairs exercise capacity through diminished cardiac output, mitochondrial dysfunction, and accelerated skeletal muscle fatigue. In patients with hypothyroidism, levothyroxine (LT4) replacement therapy has been shown to significantly restore both BMR and VO2 max toward euthyroid levels. Discussion: The bidirectional dysregulation of thyroid activity produces distinct metabolic and physiological phenotypes. The primary molecular pathways through which T3 exerts its thermogenic and hemodynamic effects include the modulation of Na+/K+ −ATPase pump activity, the induction of mitochondrial uncoupling proteins (UCPs), and the sensitization of cardiac beta-adrenergic receptors. Optimal exercise tolerance is contingent upon maintaining a euthyroid state, as both extremes of thyroid dysfunction impair physical performance through complementary but mechanistically divergent pathways. Conclusion: Thyroid hormones are indispensable for the regulation of metabolic rate and physical performance in humans. Exercise tolerance should be recognized as a critical functional outcome measure in the clinical management of thyroid disorders. Future research should prioritize investigating the impact of subclinical thyroid dysfunction on exercise physiology and establishing optimal thyroid hormone thresholds for athletic populations.
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