Influence of levothyroxine supplementation on athletic performance in subclinical hypothyroidism - a review of the literature.

Subclinical hypothyroidism (sHT), defined by elevated thyroid-stimulating hormone levels with preserved circulating thyroid hormone concentrations, may subtly affect physiological processes relevant to athletic performance. Emerging evidence suggests that levothyroxine (LT4) therapy can enhance muscle strength, mobility, and cardiopulmonary reserve, particularly in younger and middle-aged populations. This review critically examines current data on the physiological effects of levothyroxine in subclinical hypothyroidism, with a focus on its potential implications for athletic performance. The most consistent findings relate to cardiac physiology: LT4 therapy reverses diastolic dysfunction, reduces systemic vascular resistance, and enhances cardiopulmonary reserve, suggesting that functional cardiac abnormalities in sHT are largely reversible. In contrast, effects on energy metabolism remain limited. Patients with sHT exhibit impaired exercise tolerance, altered substrate utilization, and increased lactate accumulation, and these disturbances are only partially ameliorated by LT4. Overall, LT4 therapy in sHT shows promise in improving selected physiological parameters, yet evidence for meaningful enhancement of overall exercise capacity or athletic performance remains weak. Until robust, athlete-specific data are available, treatment decisions should continue to follow established clinical criteria rather than anticipated performance gains.