Iodine Deficiency and Excess Coexist in China and Induce Thyroid Dysfunction and Disease: A Cross-Sectional Study

Background: In spite of the salt iodization, iodine deficiency disorders (IDD) have not been sustainably eliminated in China. There are coastal areas with low iodized salt coverage rates (iodine nutrition is inadequate) and other areas with excessive amounts of iodine in the drinking water. Objective: This study aimed to clarify the association of iodine deficiencies resulting from a low coverage rate of iodized salt, excess iodine intake from drinking water with thyroid function and disease in adults. Design: A cross-sectional study was conducted in adults in different iodine nutrition areas in three provinces in China. Results: The prevalence of thyroid nodules was 15.52%, 8.66% and 22.17% in the iodine excess, sufficient and deficient groups, respectively. The prevalence of subclinical hypothyroidism was 20.09%, 10.41%, and 2.25% in the excess, sufficient and deficient iodine groups, respectively. The prevalence of subclinical hyperthyroidism and overt hyperthyroidism in the iodine deficient group was higher than that in the iodine excess group (chi(2) = 9.302, p = 0.002) and iodine sufficient group (chi(2) = 7.553, p = 0.006). Thyroid-stimulating hormone (TSH) was significantly correlated with excess iodine intake (beta = 1.764, P = 0.001) and deficient iodine intake (beta = -1.219, P = 0.028). Conclusions: Thyroid nodules are more likely to be present in the iodine excess and deficient areas than in the iodine sufficient areas. Subclinical hyperthyroidism and overt hyperthyroidism are more likely to be prevalent in the iodine deficient areas than in the iodine excess or sufficient areas. Subclinical hypothyroidism is more likely to be prevalent in the high iodine intake areas than in the iodine deficient or sufficient areas. Median TSH may be deemed as an alternative indicator for monitoring the iodine nutrition status of the adult population in iodine excess and deficient areas.