Assessment of Joint Impact of Iodine, Selenium, and Zinc Status on Women's Third-Trimester Plasma Thyroid Hormone Concentrations

Background Iodine is essential for synthesizing thyroid hormones, but other micronutrients are also required for optimal thyroid function. However, there is a lack of data on combined micronutrient status in relation to thyroid hormones in pregnancy. Objectives We aimed to assess the joint associations of iodine, selenium, and zinc status with plasma concentrations of thyroid hormones and thyroid-stimulating hormone (TSH) in pregnancy. Methods We included 531 pregnant women (aged 22-40 y) participating in a Swedish birth cohort who provided blood and spot urine samples in gestational weeks 27-33 (mean: 29). Associations of urinary iodine concentration (UIC), plasma selenium concentration, and plasma zinc concentration (measured by inductively coupled plasma mass spectrometry) with plasma hormone concentrations [total and free thyroxine (tT4, fT4), total and free triiodothyronine (tT3, fT3), and TSH] were explored with Bayesian kernel machine regression (BKMR; n = 516; outliers excluded) and multivariable-adjusted linear regression (n = 531; splined for nonlinear associations). Results Median (IQR) micronutrient concentrations were 112 mu g/L (80-156 mu g/L) for UIC, 67 mu g/L (58-76 mu g/L) for plasma selenium, and 973 mu g/L (842-1127 mu g/L) for plasma zinc; the former 2 median values were below recommended concentrations (150 mu g/L and 70 mu g/L, respectively). Mean +/- SD TSH concentration was 1.7 +/- 0.87 mIU/L, with 98% < 4 mIU/L. BKMR showed a positive trend of joint micronutrient concentrations in relation to TSH. Plasma zinc was most influential for all hormones but tT3, for which plasma selenium was most influential. In adjusted linear regression models, zinc was positively associated with tT4, tT3, and TSH, and <1200 mu g/L also with fT4 and fT3. Selenium was inversely associated with fT3, and Conclusions Pregnant women's plasma TSH concentrations in the early third trimester increased with increasing joint status of iodine, selenium, and zinc. Zinc and selenium were more influential than iodine for the hormone concentrations. Multiple micronutrients need consideration in future studies of thyroid hormone status.