The Effect of Baseline Thyroid Function on Weight Loss Outcomes in Euthyroid Individuals Undergoing Full Meal Replacement Therapy

Abstract Background Thyroid function plays a key role in regulating energy metabolism and thermogenesis, with thyroid dysfunction closely linked to alterations in body weight and composition. However, there is a lack of data on the effect of baseline thyroid function on weight loss outcomes in euthyroid individuals.Methods This study constitutes a secondary analysis utilizing prospectively collected data from a cohort study comprising individuals living with obesity (BMI > 30kg/m² or ≥ 27 kg/m² with comorbidities) and normal thyroid function participating in a weight management program, which incorporates full meal replacement therapy (FMR). The primary objective was to examine the association between baseline thyroid function and weight loss (WL) outcomes 6-week post-FMR initiation.Results A total of 1078 participants were included in the study: 67% female, aged 45.4±10.9 years, 64% had type 2 diabetes with an initial BMI of 45.0±7.6 kg/m², and a baseline TSH and fT3 levels of 2.0±0.8 mIU/L and 4.5±2.6 pmol/L respectively. 6-week post-FMR initiation, there was significant correlation between the amount of WL and TSH levels (β:-0.473 IC₉₅[-0.796; -0.150]). The percentage of WL between extreme TSH quantiles (Q1-Q5) were 8.1±1.8% vs 7.3±1.6% (p < 0.001). No correlation was found between WL and TSH levels at 12 weeks and fT3 levels at 6 and 12 weeks.Conclusion Within a cohort of euthyroid individuals living with obesity undergoing FMR, lower baseline TSH levels, not fT3 levels, were predictive of greater weight loss at 6-week. These findings suggest that this parameter might be an important weight loss outcomes predictive factor for euthyroid individuals with obesity.