Serum iodine levels appear to be associated with urinary iodine concentrations and thyroid function in pregnant women, according to study findings published in Clinical Endocrinology. The study results also showed that low concentrations of serum iodine were associated with a greater risk for iodine deficiency and hypothyroxinemia, while high serum iodine concentrations were linked to thyrotoxicosis.

Iodine metabolism and thyroid function are substantially altered during pregnancy and the risk for iodine deficiency is increased because of the higher iodine requirements in pregnant women. Prior research showed that serum iodine concentration is associated with iodine status in pregnant women, as well as with thyroid function in nonpregnant women, but the relationship between serum iodine concentration and thyroid function has not been assessed in pregnant women. In this study, researchers evaluated the association of serum iodine with urinary iodine concentrations and thyroid function in 1099 pregnant women who were treated at a single center in China. Serum free triiodothyronine, free thyroxine, thyroid-stimulating hormone, thyroglobulin, thyroid peroxidase antibody, thyroglobulin antibody, and iodine concentration were measured, as well as urinary iodine and creatinine.

The study results showed that median urinary iodine concentration was 156 μg/L and median serum iodine concentration was 108 μg/L. Serum iodine concentration positively correlated with urinary iodine concentration (r=.12; P <.001), free triiodothyronine (r =.22; P <.001), and free thyroxine (r=.50; P <.001), but was inversely correlated with thyroid-stimulating hormone (r=-0.14; P <.001). Participants in the 10th percentile for serum iodine concentration (<79.9 μg/L) demonstrated a greater risk for hypothyroxinemia vs women with higher serum iodine concentration (odds ratio [OR], 2.44; 95% CI, 1.31-4.75; P =.005). Conversely, women with serum iodine concentration above the 90th percentile (>138.5 μg/L) had a higher tendency to develop thyrotoxicosis than women with lower serum iodine concentration values (OR, 13.52; 95% CI, 4.21-43.36; P <.001).

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A limitation of this study, wrote the researchers, was that “serum iodine [levels] of pregnant women in [iodine-deficient] and excessive areas were not examined in our study. Additional studies will be needed to assess these associations across a wider range of iodine intakes.”

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Reference

Pan Z, Cui T, Chen W, et al. Serum iodine concentration in pregnant women and its association with urinary iodine concentration and thyroid function [published online February 6, 2019]. Clin Endocrinol (Oxf). doi:10.1111/cen.13945