Increased incidence of Hashimoto's thyroiditis in selenium deficiency: a prospective six-year cohort study

In 2015, we reported an increased prevalence of thyroid disease in a county of low habitual selenium (Se) intake in comparison to a neighbouring county with higher intake in a cross-sectional survey in Shaanxi Province, China. To explore longitudinal effects of low Se status, a prospective cohort study was conducted in the same area from 2013 to 2019, and TPO-autoantibodies (TPO-Ab) and disease incidence were compared.A total 1,254 subjects from 1,500 re-invited participants were successfully enrolled. Venous blood, fingernails, and urine samples were collected and analysed to evaluate thyroid status, TPO-Ab, serum Se, and urinary iodine. Diagnosis of Hashimoto's thyroiditis (HT) was based on elevated TSH, presence of TPO-Ab, and ultrasound characteristics. Se deficiency was categorized using a serum concentration of 80 µg/L as threshold, and tested by logistic regression for a relationship to TPO-Ab and HT.Se deficiency was observed in 46.2% of participants from the adequate-Se county (Ziyang) and in 89.7% from the low-Se county (Ningshan). Se concentrations in fingernails differed strongly with residency (Ziyang vs. Ningshan; 678.7 vs. 364.3 μg/kg; Z=-9.552, P<0.001). Newly diagnosed HT in Ziyang was less frequent than in Ningshan (0.09% vs. 0.31%; χ2=4.350, P=0.037). The conversion rate to seropositive TPO-Ab was 10.2% in Ningshan vs. 5.6% in Ziyang. Excluding iodine as confounding factor, low-Se was confirmed as a risk factor for HT (relative risk(95%CI); 3.65(1.03-12.90), P<0.05).The data indicate an increased incidence of TPO-Ab seroconversion with low Se supply and support the hypothesis that Se deficiency contributes to HT as a modifiable risk factor.