The associations between resting and total energy expenditure, physical activity, and thyroid hormone levels in adult females

Introduction: Physical activity (PA) costs energy. However, recent theory suggests the relationship of increasing PA to total energy expenditure (TEE) is non-linear because high levels of activity suppress resting metabolism mediated via changes in thyroid hormones (THs; T3- triiodothyronine, T4- thyroxine, fT3- free triiodothyronine, fT4 - free thyroxine, TSH - thyroid stimulating hormone). Method: We conducted a cross-sectional study on 38 euthyroid females (NCT06377943) to examine the relationship between PA, resting metabolic rate (RMR), and TEE. Multiple linear regression and Bootstrapped structural equation model were used to assessed whether THs statistically accounted for the association between PA and RMR. Results: Bootstrapped structural equation models indicated that PA was significantly associated with lower TH levels (β = -4.33, 95% CI [-8.51, -1.70], p = 0.011), while the association between THs and RMR was small and non-significant (β = 0.043, 95% CI [-0.02, 0.10], p = 0.16), resulting in a non-significant indirect effect (β = -0.19, 95% CI [-0.67, 0.08], p = 0.31). In multiple regression plots, physical activity energy expenditure (PAEE) and PA showed a trend toward significance (p = 0.07) when THs were added. Conclusion: Overall, although PA was associated with modest changes in THs, these changes did not correspond to any detectable decrease in RMR.