Cross-sectional association of TRAb at diagnosis with FT4 levels. (a) Association of TRAb with FT4. We used linear regression models with restricted cubic splines. Predicted means of FT4 (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 0.0008, 0.5, 0.9, 0.6, and 0.3, respectively. (b) Association of TRAb with FT4 among age tertiles.