Serum lipids profile in relation to free thyroxine and the effect of levothyroxine treatment on lipids in patients with isolated hypothyroxinemia during pregnancy: a single-centre retrospective study

Backgroud Thyroid function was widely considerated as one of regulators in lipids metabolism. Studies on lipids metabolism in the pregnant women with low FT 4 levels are few and inconclusive. Because it is not known whether maternal FT 4 deficiency during pregnancy is associated with adverse metabolic parameters, our study aimed to further investigate the association between maternal FT4 levels and lipids profile and the therapeutic effects of levothyroxine treatment(L-T 4 ) on these lipids profile. Methods: This retrospective study comprised 164 IH patients(FT 4 levels below the 5 th percentile with normal serum thyroid-stimulating hormone(TSH) levels according to the gestational-specific reference range established by Zhejiang Provincial People's Hospital previously) and 208 euthyroidism(CON group) who had regular antenatal checks at this hosptial from 2019 to 2021. IH patients were divided into levothyroxine treatment group(Treatment group, n=77) and dietary iodine supplement group(Non-Treatment group, n=87) owing to the hospital’s treatment policy and clinical experience. The intervention lasted at least 4 weeks. Metabolic parameters, including thyroid function and lipids parameters, were collected at least two times before and after the intervention. Several general characteristics were included likewise, such as maternal age, history of abortion, progestation body mass index(BMI) and gestational weight gain(Fig. 1). Results: Compared with the CON group, the IH group displayed higher levels of dyslipidaemia, reflected in elevated total cholesterol(TC), triglycerides(TG), low-density lipoprotein cholesterol(LDL-C), and apolipoprotein B(Apo B). In IH patients, a significant and negative correlation was found between FT 4 and TG levels, which remained after adjusting for progestation BMI. After levothyroxine treatment, the Treatment group demonstrated a significantly slower rate of progression of hypercholesterolaemia during pregnancy. In addition, there was a relationship between the therapeutic effect and the degree of disease, with the main factors being FT 4 , TSH and TG levels prior to starting treatment. Conclusions: Low FT 4 level status is associated with elevated blood lipids. Thyroid function and lipid levels could be improved to some extent after medical intervention in IH patients.