Body composition indices align with thyroid variation in euthyroid Saudi adults: A cross-sectional study

Objective Although associations between obesity and thyroid function have been widely reported, few studies have examined euthyroid (clinically normal) adults using both bioelectrical impedance-derived body composition measures and clinically accessible central adiposity markers in the same analysis, particularly Saudi adults in sex-stratified models. We examined the associations between thyroid function (evaluated using thyroid-stimulating hormone, free thyroxine, and free triiodothyronine) and body composition in Saudi adults, considering sex-specific patterns. Methods We conducted a cross-sectional analysis including 551 euthyroid adults (aged ≥18 years, 59% men). Anthropometric characteristics and body composition parameters were evaluated. Serum thyroid-stimulating hormone, free triiodothyronine, and free thyroxine levels were measured. Correlations and logistic regression analyses were used to assess associations. Participants were categorized as having high thyroid-stimulating hormone, low free thyroxine, or low free triiodothyronine levels for an assessment of their obesity risk. Results Low free thyroxine levels were associated with higher odds of obesity, as classified using body mass index (odds ratio = 1.85; 95% confidence interval, 1.09–3.10) and waist circumference (odds ratio = 2.68; 95% confidence interval, 1.21–6.20). High thyroid-stimulating hormone levels alone demonstrated a limited association to obesity risk. However, in women, high thyroid-stimulating hormone levels were strongly associated with central obesity. In men, low free thyroxine level was the more prominent predictor, associated with increased odds of elevated waist circumference. Free triiodothyronine levels were positively correlated with lean body mass and negatively with fat percentage, as indicated by higher free triiodothyronine levels in individuals with more muscle and less fat. Conclusions Greater adiposity was associated with higher thyroid-stimulating hormone and lower free thyroxine levels in our population. These relationships differed by sex; the association of elevated thyroid-stimulating hormone levels with adiposity was more pronounced in women, whereas a stronger inverse free thyroxine–obesity link was observed in men.