Preconception thyroid function in relation to pregnancy lipids among women attending a fertility center.

Context: Thyroid hormones play an essential role in lipid metabolism. The association of thyroid function and dyslipidemia in women has been mostly studied in the general population, and epidemiological evidence during pregnancy is scarce.

Objective: We investigated the association of preconception thyroid function and autoimmunity with serum lipid biomarkers during pregnancy among 142 pregnant women enrolled in the Environment and Reproductive Health (EARTH) Study (2005 - 2017) who were seeking treatment at a fertility center.

Methods: Before conception, we measured biomarkers of thyroid function and autoimmunity including thyroid stimulating hormone (TSH), free thyroxine (fT₄), total thyroxine (TT₄), free triiodothyronine (fT₃), total triiodothyronine (TT₃), thyroid peroxidase antibodies (TPOAb), and thyroglobulin antibodies (TgAb). During pregnancy, we assessed lipid biomarkers including serum levels of total triglycerides (TG), total cholesterol (TC), high-density lipoprotein (HDL), non-HDL, and low-density lipoprotein (LDL) cholesterol. Linear regression models were used to study these associations while adjusting for age, body mass index, physical activity, multifetal gestation, infertility diagnosis and trimester of lipid measurement.

Results: We observed a negative association between preconception serum TT₃ and HDL cholesterol levels during pregnancy (β estimate = -8.6, 95% CI = -16.2, -1.1), and a smaller negative association for TT₄ with HDL (β estimate = -0.2, 95% CI = -4.0, 0.0). When stratified by trimester, these associations were stronger for those with lipids measured in the 3^rd trimesters but were not observed for lipids measured in the 1^st and 2^nd trimesters. We also found that each 0.1 unit increase in the preconception fT₃: fT₄ ratio was associated with higher total cholesterol (β estimate = 32.4, 95% CI = 1.93, 62.6), non-HDL (β estimate = 35.8, 95% CI = 5.63, 65.9) and LDL (β estimate = 32.1, 95% CI = 4.48, 59.7) cholesterol in 3^rd trimester, but no association for those with lipids measured in earlier trimesters. No other associations were found for the other examined exposures and outcomes.

Conclusions: This study showed that some preconception biomarkers of thyroid function were associated with serum lipid levels during pregnancy. These results highlight the importance of thyroid function during the preconception window, and its potential impact on cholesterol levels at different time periods in the pregnancy.