Association of Thyroid Hormones with Basal Metabolism and Body Composition in Women of Reproductive Age

Background: Thyroid hormones are essential regulators of energy expenditure, thermogenesis, and body composition. Although overt thyroid dysfunction is well known to alter basal metabolic rate (BMR) and body mass, emerging evidence suggests that even hormonal variations within the reference range may exert measurable effects on metabolic and body composition profiles. Women of reproductive age represent a population particularly sensitive to hormonal oscillations due to the interplay between endocrine, reproductive, and cardiometabolic health. Objective: The study aimed to a) analyze thyroid hormone levels (TSH, FT3, FT4) alongside anthropometric and body composition parameters in women of reproductive age; b) examine thyroid hormone levels, BMR, and body composition parameters across age groups; and c) investigate associations of thyroid hormones with BMR, body composition components, and unfavorable body composition patterns (visceral adiposity, elevated metabolic age, obesity) as well as metabolic indicators.. Methods: A total of 117 women aged 18–45 years were included in this cross-sectional, observational study conducted in Bosnia and Herzegovina between September 2023 and November 2024. Thyroid hormone levels were measured using electrochemiluminescence assays, while body composition was assessed by bioelectrical impedance analysis. Statistical analyses included descriptive methods, Pearson’s correlation, and Chi-square testing, with significance set at p<0.05. Results: TSH showed significant positive associations with fat-free mass, muscle mass, and BMR (p<0.05). FT3 was inversely correlated with metabolic age and visceral fat, while FT4 demonstrated weak negative associations with fat-free mass and metabolic age (p<0.05). Significant age-related differences were observed in fat percentage, fat mass, BMI, visceral fat, and metabolic age, with the most unfavorable profiles in women aged 31–40 years. Conclusion: Thyroid hormones, even within the reference range, are associated with body composition and metabolic parameters in women of reproductive age. Their role as early indicators of unfavorable metabolic patterns highlights potential implications for reproductive and cardiovascular risk assessment.