Objectives: Thyroid function has long been associated with body composition in the general population. Little is known regarding the association among individuals with obesity. We aim to study this association in individuals with obesity with and without a combined lifestyle intervention (CLI).

Methods: Participants with body mass index (BMI) ≥30kg/m², were included from an observational cohort, the Rotterdam Study, and from the Obesity Center CGG, Erasmus Medical Center, including individuals receiving a CLI. Thyroid-stimulating hormone (TSH) and free thyroxine (FT4) were measured at baseline. Body composition measurements including BMI, waist circumference (WC), and body fat percentage measured by dual-energy X-ray absorptiometry were available at baseline and during follow-up. Multivariable linear mixed models were used, adjusting for age, sex, diet, smoking, and physical activity.

Results: We included 1800 participants from the Rotterdam Study (mean age 64.1 years, 61.4% women) and 172 participants from the Obesity Center CGG (mean age 43.0 years, 74.4% women). Among individuals receiving a CLI, per one unit increase of log-transformed TSH was associated with 0.65 (0.04-1.26) kg/m² greater annual BMI loss and 1.96 (0.08-3.85) cm greater annual WC loss, while per one unit increase of FT4 in pmol/L was associated with 0.14 (0.02-0.25) kg/m² less annual BMI loss and 0.42 (0.07-0.77) cm less annual WC loss. Among individuals aged <65 years from Rotterdam Study, per one unit increase of log-transformed TSH was associated with 0.04 (0.001-0.08) kg/m² greater annual BMI loss, while no association was indicated for individuals aged ≥65 years. We did not identify any association between thyroid function and changes in body fat percentage either among individuals with a CLI intervention or those from Rotterdam Study.

Conclusion: Higher TSH concentrations might implicate higher efficacy of lifestyle interventions on obesity. Our study provides evidence against thyroid hormone supplementation among individuals with obesity having hyperthyrotropinaemia.