Effect of thyroid hormones on Basal Metabolic Rate (BMR) in adults

Background: Thyroid hormones are major regulators of energy expenditure (BEE) and basal metabolic rate (BMR), by affecting mitochondrial function, substrate utilization and thermogenesis. Changes in thyroid function may result in metabolism imbalances, whereas adipokines and leptin specifically interact with thyroid pathways to influence energy expenditure. Although there have been many studies to date, literature on adult populations using thyroid hormone, leptin and modern metabolic biomarkers is limited. Objective: We sought to determine the effects of thyroid hormones on BMR in normal adult subjects and to assess the associations between BMR, levels of leptin, body composition and novel metabolic biomarkers. Patients and methods: In this prospective observational study, 250 adults (of either sex) aged 18-65 years were selected from Tikrit Teaching Hospital in Iraq from January to November 2025. A comprehensive medical history was obtained, and all participants underwent routine assessment of anthropometric measures impedance analysis for body composition (BIA), indirect calorimetry for basal metabolic rate (BMR) and blood examination that included TSH, FT4, FT3 total T4 and T3 levels evaluation, leptin, adiponectin, FGF21 as well lipid profile, fasting glucose and insulin levels along with thyroid autoantibodies. Potential associations between variables were measured using correlation and multivariate regression analyses. Results: Free T3 was the greatest positive correlation with BMR followed by FT4 and was negatively correlated with TSH. BMR was higher in males and positively correlated with lean body mass, while leptin levels were inversely correlated with BMR and positively associated with the percentage of fat. Those with low levels of the hormone also had higher amounts of fat in their blood and increased resistance to insulin. The new biomarkers, adiponectin and FGF21, represented metabolic adaptation and insulin sensitivity. Free T3, lean body mass, sex and leptin emerged as independent predictors of BMR in multivariate regression analyses. Conclusions: Thyroid hormones - mainly fT3 - exert a major effect upon BMR in adults, the latter being variably modulated by body composition and leptin. Measurements of hormones, metabolites and body composition improve comprehension of energy homeostasis and might influence the treatment management of thyroid-associated metabolic disorders.