Correlation between iodine intake and thyroid disorders: a cross-sectional study from the South of China

Great changes have taken place in the incidence of thyroid diseases since the implementation of universal salt iodization (USI). However, the high incidence of thyroid diseases caused by the high iodine intake has been contentious. The aim of this study was to investigate the relationship between iodine intake and thyroid diseases through the comparison of urine iodine concentration (UIC) between patients with thyroid diseases and healthy volunteers and to assess the status of iodine intake among the residents. From November 2013 to May 2014, 905 patients who underwent thyroid surgeries and 272 subjects of healthy controls were enrolled and were divided into two groups: the case group and the control group, respectively. Levels of thyroid hormones and thyroid autoantibodies in serum from blood were analyzed among all the patients. UIC and thyroid B ultrasounds were performed on each participant. The median urinary iodine (MUI) concentration was 184.5 and 169.6 μg/L for case group and control group, respectively (P = 0.003). Significant differences of the MUI were found between healthy controls and patients with Hashimoto's thyroiditis (MUI = 221.3 μg/L), nodular goiter (MUI = 193.5 μg/L), multiple nodules (MUI = 185.9 μg/L), nodule diameter ≥1 cm (MUI = 194.4 μg/L), hyperthyroidism (MUI = 258.7 μg/L), thyroid peroxidase antibody (TPOAb) (+), and thyroglobulin antibody (TGAb) (+) (MUI = 196.4 μg/L), and P values were 0.003, 0.000, 0.002, 0.000, 0.000, and 0.001, respectively. The susceptibility of the thyroid diseases among normal people was significantly associated with female sex (odds ratio (OR) = 3.3), older age (OR = 2.1), and high iodine intake (OR = 1.3). In conclusion, high iodine intake was likely to lead to the occurrence of thyroid diseases, such as Hashimoto thyroiditis, nodular goiter, and hyperthyroidism, through a long-term mechanism. USI should continue to be carried out and individual UIC detection was recommended for the disequilibrium of the iodine nutritional status among normal people.