Association between thyroid function tests and anti-thyroid peroxidase (TPO) antibodies in pregnancy

Anti-thyroid peroxidase (TPO) antibodies are self-developed antibodies that target the membrane-associated hemoglycoprotein, thyroid peroxidase, of the thyroid cells. They are markers of thyroid autoimmunity and are commonly performed in the work-up of Hashimoto thyroiditis (TPO antibodies prevalence: 90 %) and Grave’s disease (TPO antibodies prevalence: 60–80 %). In patients with subclinical hypothyroidism, the finding of TPO antibodies confers an increased risk of progression to overt hypothyroidism. In pregnancy, the presence of TPO antibodies are associated with adverse outcomes, including miscarriage, preterm delivery, subfertility, perinatal mortality, large for gestational age and low birth weight infants and post-partum thyroiditis [1]. A recent meta-analysis has shown that elevated TPO antibodies in pregnant women are associated with significantly increased risk of preterm birth (odds ratio 2.07) and miscarriage (odds ratio 1.80 in case–control studies; odds ratio 3.90 in cohort studies) [2]. The precise mechanism underlying these complications is unclear at present and is an area of research of great activity. In the same meta-analysis, serum thyroid-stimulating hormone (TSH) was found to be 0.51 mIU/L higher in pregnant women with positive TPO antibodies compared to those with negative TPO antibodies. This finding lends support to the hypothesis that women with positive TPO antibodies have relative hypothyroidism. However, the report found insufficient data on free thyroxine (FT4) and free triiodothyronine (FT3) concentrations for meta-analysis to support this observation. The effect of TPO antibodies on intra-partum maternal thyroid function is less well described. Here, we explored the relationship between thyroid antibodies and thyroid hormones in different trimesters, in a large cohort of clinically euthyroid pregnant women.