Maternal Adiposity, Smoking, And Thyroid Function In Early Pregnancy

Objective: A high activity of the deiodinase type 2 has been proposed in overweight, obese, and smoking pregnant women as reflected by a high triiodo thyronine (T3)/thyroxine (T4) ratio. We speculated how maternal adiposity and smoking would associate with different thyroid function tests in the early pre gnancy.Design: Cross-sectional study within the North Denmark Region Pregnancy Cohort.Methods: Maternal thyroid-stimulating hormone ( TSH), total T4 (TT4), total T3 ( TT3), free T4 (fT4), and free T3 ( fT3) were measured in stored blood samples (median gestational week 10) by an automatic immunoassay. Results were linked to nationwide registers, and live-birth pregnancies were included. The associations between maternal adiposity (overweight or obese), smoking, and log-transformed TSH, fT3/fT4 ratio, and TT3/ TT4 ratio were assessed using multivariate linear regression and reported as adjusted exponentiated beta coefficient (alpha beta) with 95% CI. The adjusted model included maternal age, parity, origin, week of blood sampling, and diabetes.Results: Altogether 5529 pregnant women were included, and 40% were cl assified with adiposity, whereas 10% were smoking. Maternal adiposity was associated with higher TSH (alpha beta 1.13 ( 95% CI 1.08- 1.20)), whereas maternal smoking was associated with lower TSH in the early pregnancy (0.875 (0.806-0.950)). Considering the T3/T4 ratio, both maternal adiposity ( fT3/fT4 ratio: 1.06 (1.05- 1.07); TT3/TT4 ratio: 1.07 (1.06-1.08)) and smoking (fT3/fT4 ratio: 1.07 (1.06-1.09); TT3/TT4 ratio: 1.10 (1.09-1.12)) were associated with a higher ratio.Conclusions: In a large cohort of Danish pregnant women, adiposity and smoking showed opposite associations with maternal TSH. On the other hand, both conditions were associated with a higher T3/T4 ratio in early pregnancy, which may reflect altered deiodinase activity.