Evaluation of Iodine Nutritional Status Among Pregnant Women in China

Background: Remarkable achievements have been made for over two decades by implementing a universal salt iodization policy in China. However, changes in the standards and manufacturing of iodized salt have made it necessary to regularly monitor iodine intake and thyroid function in the population. Therefore, we aimed to evaluate iodine nutritional status using thyroid function and urinary iodine levels in pregnant women via a national survey. Methods: Participants included pregnant women enrolled in the 2015 Chinese Adults Chronic Diseases and Nutrition Surveillance (CACDNS). Urinary iodine concentration (UIC) was measured using arsenic and cerium catalysis spectrophotometry. Plasma levels of thyrotropin (TSH), fT4 (free thyroxine), thyroid peroxidase antibodies (TPO-Ab), and thyroglobulin antibodies (TG-Ab) were determined using an automated chemiluminescence immunoassay analyzer. Results: UIC was analyzed in a total of 6173 urinary samples collected from pregnant women residing in 31 provinces of China. The median urinary iodine concentration was 146 μg/L. Pregnant women with UIC >250 μg/L had higher TSH levels than those with UIC 120-149 and 150-249 μg/L. After excluding women with positive TPO-Ab and TG-Ab, a total 2097 plasma samples collected from pregnant women during three periods were analyzed for TSH and fT4. The frequency of normal thyroid function in this study was >85%. The most common type of abnormal thyroid function, subclinical hypothyroidism, was similar to the 2010-2012 China Nutrition and Health Surveillance (CNHS), but its prevalence decreased to 6.8% compared with that in the 2010-2012 CNHS. Compared with pregnant women who had UIC <250 μg/L, the prevalence of subclinical hypothyroidism was significantly increased in those with UIC >250 μg/L. Even with UIC levels 120-149 μg/L, the iodine status of Chinese pregnant women in the 2015 CACDNS is acceptable. Conclusions: The iodine status of pregnant women is generally adequate, but urinary iodine levels are close to the cutoff for suitable iodine status (150 μg/L). With noniodized salt widely available, continued improvement in monitoring of iodine nutritional status and thyroid function is important in this population.