Introduction <p>Obesity has become a significant global public health issue that seriously threatens the health of more than one billion individuals worldwide. The weight-adjusted waist index (WWI) is a novel adiposity indicator calculated as waist circumference (cm) divided by the square root of body weight (kg). However, the association between WWI and thyroid function has not been reported to date. This study aims to investigate the relationship between WWI and thyroid function among U.S. adults.</p> Methods <p>A total of 7,642 participants were enrolled in this study based on the National Health and Nutrition Examination Survey (NHANES) dataset from 2007 to 2012. Weighted statistical analyses were performed and three regression models were constructed sequentially, namely Model 1 (unadjusted), Model 2 (adjusted for age, sex, race, and survey cycle), and Model 3 (further adjusted for educational level, marital status, smoking status, alcohol consumption, diabetes mellitus, urinary iodine concentration, and poverty income ratio [PIR]). Stratified analyses and interaction effect tests were also conducted. Additionally, a retrospective validation cohort comprising 60 health check-up patients was extracted from Zhongshan Hospital Affiliated to Xiamen University between December 2024 and December 2025 to evaluate the association between WWI and free triiodothyronine (FT3) across WWI quartiles. The thyroid function indicators assessed included free triiodothyronine (FT3), free thyroxine (FT4), total triiodothyronine (TT3), total thyroxine (TT4), thyroid-stimulating hormone (TSH), thyroid peroxidase antibody (TPOAb), thyroglobulin antibody (TgAb), and thyroglobulin (Tg).</p> Results <p>A total of 7,642 participants were included in this study. A positive correlation between WWI and FT3 levels was observed in Models 2 and 3, and this positive association persisted in both models after stratifying WWI into quartiles. The only exception was the WWI Q2 stratum in Model 3 where the 95% confidence interval (CI) for the association between WWI and FT3 included zero, indicating no statistical significance. Further stratified analyses revealed that the positive WWI-FT3 association was primarily evident in males, individuals under 50 years of age, and those of other Hispanic ethnicity. The positive WWI-FT3 correlation identified in NHANES was successfully replicated in our institutional validation cohort.</p> Conclusion <p>A positive association between WWI and FT3 is suggested; however, further investigations are warranted to confirm this observation.</p>