Cross-sectional association of TRAb at diagnosis with FT3 levels. (a) Association of TRAb with FT3. We used linear regression models with restricted cubic splines. Predicted means of FT3 (black lines) with 95% CIs (gray areas) were plotted against TRAb, adjusting for age, sex, race, smoking status, and TPOAb. P values for interactions of TRAb with age, sex, race, smoking status, and TPOAb levels were 0.0006, 0.1, 0.7, 0.7, and 0.1, respectively. (b) Association of TRAb with FT3 among age tertiles.