Body composition changes across a spectrum of hypothyroidism severity - a systematic review and meta-analysis

This systematic review and meta-analysis aimed to investigate the impact of clinical hypothyroidism (CH) and subclinical hypothyroidism (SCH) on anthropometric parameters, such as body mass index (BMI), body weight (BW), waist circumference (WC), Hip Circumference (HC), Waist Circumference (WC) and Waist-Hip ratio (W/H). Databases including PubMed, Web of Science, Scopus, and EMBASE were searched for observational studies on hypothyroidism and body composition. The review included 38 studies (54 comparison groups, 49,427 individuals). Effect sizes (ES) with 95% confidence intervals (CI) were used to measure the effects of CH and SCH on body composition compared to healthy controls and between themselves. Effect sizes were defined as negligible (< 0.2), small (0.20-0.49), moderate (0.50-0.79), and large (> 0.8). CH showed large increases in BMI (ES = 1.092; CI: 0.755, 1.429) and BW (ES = 0.897; CI: 0.404, 1.389), and moderate increases in WC (ES = 0.759; CI: 0.419, 1.099) and fat mass (ES = 0.609; CI: 0.051, 1.167). SCH showed moderate increases in BMI (ES = 0.596; CI: 0.403, 0.789) and BW (ES = 0.712; CI: 0.287, 1.138), a small increase in WC (ES = 0.298; CI: 0.141, 0.454), with negligible fat mass changes (ES=-0.055; CI: - 0.760, 0.649). Analysis of SCH by thyroid-stimulating hormone (TSH) levels revealed greater impacts on body composition with increasing TSH. In turn, BMI (ES=-0.082; CI: - 0.577, 0.413) and BW (ES = 0.054; CI: - 0.441, 0.550) showed no significant changes at near-normal TSH (4.0-4.9 mIU/L). Moderate TSH elevation (5.0-10.0 mIU/L) led to moderate increases in BMI (ES = 0.584; CI: 0.343, 0.825) and BW (ES = 0.659; CI: 0.245, 1.073), with a small WC increase (ES = 0.271; CI: 0.077,0.465). TSH > 10.0 mIU/L resulted in large increases in BMI (ES = 1.426; CI: 0.614, 2.238) and BW (ES = 1.942; CI: 1.550, 2.334), along with a small WC increase (ES = 0.271; CI: 0.077, 0.465). The comparison regarding BMI, WC and BW showed no differences between CH and SCH. Both CH and SCH are associated with changes in body composition, mainly BMI, WC, and BW, which may contribute to metabolic risk. Body composition worsens in SCH as TSH levels rise, and stronger effects are evidenced in females and symptomatic individuals.