Long-Term Variations of Antithyroperoxidase Antibodies and its Clinical Significance

Various cut-offs have been proposed for thyroid peroxidase antibodies (TPOAb) positivity. Considering that the long-term trend of TPOAb levels and its positivity incidence is not clearly understood, we conducted the current study to determine the longitudinal variations of TPOAb in a population-based cohort study. We followed 5783 individuals of Tehran Thyroid cohort Study (TTS) for 10 years (4 phases). After exclusions, data of 3493 euthyroid participants remained for analyses. The baseline prevalence rates of TPOAb positivity were 19.8, 17, and 11.4% and the annual incidence rates (95% CI) of TPOAb positivity were 8.53 (8.29-8.77), 7.59 (7.37-7.80) and 6.79 (6.60-6.98) per 1000 persons for the 3 proposed cut-offs of 14.77, 18.38, and 40 U/l; respectively. Although a slightly increasing trend was observed for TPOAb levels (p=0.001) and its conventional positivity (TPOAb>40U/l), the recently proposed cut-offs of 14.77 and 18.38 U/l showed constant TPOAb positivity over 10 years. The time trends of the TPOAb levels among younger participants were significantly different from older participants (time×age effect p=0.004), with the former having an increasing trend and the latter, a relatively decreasing trend. Although the prevalence of TPOAb positivity was significantly (p<0.001) higher among women as compared to men, the longitudinal changes of TPOAb were similar in men and women. TPOAb positivity along with TSH values between 2.5 and 5.0 mU/l or free T4 values between 0.93 and 1.7 ng/dl exerted a significantly increased risk of subclinical or overt hypothyroidism. In an iodine sufficient population, an increasing trend in TPOAb levels was observed in line with the increasing incidence of subclinical and overt hypothyroidism.