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

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.

OBJECTIVE: To assess the joint associations of iodine, selenium, and zinc status on plasma concentrations of thyroid hormones and thyroid stimulating hormone (TSH) in pregnancy.

METHODS: We included 531 pregnant women (aged 22-40) participating in a Swedish birth-cohort who provided blood and spot urine samples in gestational week 27-33 (mean 29). Associations of urinary iodine (UIC), plasma selenium, and plasma zinc (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 non-linear associations).

RESULTS: Median (IQR) micronutrient concentrations were 112 (80-156) µg/L for UIC, 67 (58-76) µg/L for plasma selenium, and 973 (842-1127) µg/L for plasma zinc; the former two median values being below recommended levels (150 µg/L and 70 µg/L, respectively). Mean±SD for TSH was 1.7±0.87 mIU/L, with 98% below 4 mIU/L. BKMR showed a positive trend of joint micronutrient concentrations with 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 below 1200 µg/L also with fT4 and fT3. Selenium was inversely associated with fT3, and below 85 µg/L with tT3.

CONCLUSIONS: Pregnant women's plasma TSH concentrations in 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.