In the Journals Plus

Serum iodine level associated with thyroid function in pregnancy

Show Citation

February 11, 2019

Chinese women with a low serum iodine concentration during pregnancy were 2.4 times more likely to develop hypothyroxinemia vs. women with a higher iodine concentration, whereas a high iodine concentration during pregnancy was related to risk for thyrotoxicosis, according to findings published in Clinical Endocrinology.

Pregnant women are highly vulnerable to iodine deficiency, in part because of the transfer of iodine to the fetus during pregnancy and increased urinary iodine excretion, Wanqi Zhang, MD, PhD, vice dean of the School of Public Health at Tianjin Medical University, and colleagues wrote. WHO recommends that pregnant women increase their nutritional iodine intake to 250 µg per day and the American Thyroid Association recommends a 150 µg per day iodine supplement to prevent maternal iodine deficiency.

“There are many studies that reported [urinary iodine concentration] values for pregnant women, but few have focused on another biomarker of iodine nutrition, serum iodine,” the researchers wrote.

Zhang and colleagues analyzed data from 1,099 pregnant women with no history of thyroid disease who underwent routine prenatal exams at Tanggu Maternity Hospital between March 2016 and May 2017 (mean age, 28 years). Women provided spot urine and fasting blood samples to analyze urinary iodine level, as well as serum free triiodothyronine, free thyroxine, thyroid-stimulating hormone, thyroglobulin and serum iodine concentrations. Researchers used logistic regression analyses to assess whether serum iodine concentration was associated with iodine deficiency, iodine excess and thyroid dysfunction.

Within the cohort, median urinary iodine concentration was 156 µg/L and median serum iodine concentration was 108 µg/L. Ranges of serum iodine concentration were 78.6 µg/L to 178.8 µg/L for the first trimester, 67.3 µg/L to 163.8 µg/L for the second trimester and 60.2 µg/L to 144.1 µg/L for the third trimester.

Researchers found that serum iodine concentration was positively correlated with urinary iodine concentration (r = 0.12; P < .001) free T3 (r = 0.23; P < .001) and free T4 (r = 0.5; P < .001) and was inversely correlated with TSH (r = –0.14; P < .001).

“Our results demonstrate that the correlation between [serum iodine concentration] and serum [free] T4 level is stronger than that between [serum iodine concentration] and [urinary iodine concentration],” the researchers wrote. “Although the [free] T4 levels of pregnant women are typically lower than those of non-pregnant women, total T4 levels are typically higher, due to increases in thyroxine binding globulin. Therefore, it can be hypothesized that the elevated [serum iodine concentration] in pregnant women is primarily caused by the surge of thyroid hormone production and elevated total serum thyroxine.”

In logistic regression analyses, researchers also found that pregnant women with a serum iodine concentration of less than 79.9 µg/L were 2.44 times more likely to have hypothyroxinemia when compared with pregnant women with an iodine level of at least 79.9 µg/L (95% CI, 1.31-4.748). Pregnant women with a serum iodine concentration greater than the 90th percentile were at increased risk for both a urinary iodine concentration of greater than 500 µg/L (OR = 2.97; 95% CI, 1.47-6.03) and thyrotoxicosis (OR = 13.52; 95% CI, 4.21-43.36).

The researchers noted that only spot urine samples were collected, so there was a lack of 24-hour urine data, and the trimester-specific measurements were not longitudinal and were not repeated in the same participants.

“Finally, we only studied in an adequate iodine area and serum iodine of pregnant women in iodine deficient and excessive areas were not examined in our study,” the researchers wrote. “Additional studies will be needed to assess these associations across a wider range of iodine intakes.” – by Regina Schaffer

Disclosures: The authors report no relevant financial disclosures.

itj+ Infographic

itj+ Perspective