Plasma mineral (Se, Zn or Cu) concentrations in the general pregnant population, adjusted for supplement intake, in relation to thyroid function.

The current study reports on first-trimester reference ranges of plasma mineral Se/Zn/Cu concentration in relation to free thyroxine (FT4), thyrotropin (TSH) and thyroid peroxidase antibodies (TPO-Ab), assessed at 12 weeks' gestation in 2041 pregnant women, including 544 women not taking supplements containing Se/Zn/Cu. The reference range (2.5th - 97.5th percentiles) in these 544 women was 0.72 - 1.25 µmol/L for Se, 17.15 - 35.98 µmol/L for Cu, and 9.57 - 16.41 µmol/L for Zn. These women had significantly lower mean plasma Se concentration (0.94 µmol/L, SD 0.12) than those (N = 1479) taking Se/Zn/Cu supplements (1.03 µmol/L, SD, 0.14; P < 0.001) while the mean Cu (26.25 µmol/L) and Zn (12.55 µmol/L) concentrations were almost identical in these sub-groups. Women with hypothyroxinemia (FT4 below reference range with normal TSH) had significantly lower plasma Zn concentrations than euthyroid women. After adjusting for co-variates including supplement intake, plasma Se (negatively), Zn and Cu (positively) concentrations were significantly related to logFT4; Se and Cu (but not Zn) were positively and significantly related to logTSH. Women taking additional Se/Zn/Cu supplements were 1.46 (95% CI: 1.09 - 2.04) times less likely to have elevated titres of TPO-Ab at 12 weeks of gestation. We conclude that first-trimester Se reference ranges are influenced by Se-supplement intake while Cu and Zn ranges are not. Plasma mineral Se/Zn/Cu concentration are associated with thyroid FT4 and TSH concentrations. Se/Zn/Cu supplement intake affects TPO-Ab status. Future research should focus on the impact of trace-mineral status during gestation on thyroid function.