Relationship between maternal obesity and first trimester TSH in women with negative anti-TPO antibodies.

OBJECTIVE: obesity is associated with increased Thyroid-stimulating-hormone (TSH) in non-pregnant subjects, but this phenomenon has not been fully characterized during pregnancy. Our aim was to evaluate the impact of Body-Mass-Index (BMI) on first-trimester TSH in a wide cohort of pregnant women with negative anti-Thyreoperoxidase-Antibodies (AbTPO) and its implications on uterine-artery pulsatility-index (UtA-PI), a marker of early placentation.

METHODS: the study included 2268 AbTPO-negative pregnant women at their first antenatal visit. Anamnestic data, BMI, TSH, anti-nucleus-antibody (ANA) and extractable-nuclear-antigen (ENA) positivity and mean UtA-PI were collected.

RESULTS: 1693 women had normal weight, 435 were overweight and 140 were obese. Maternal age, ANA/ENA positivity, history of autoimmune diseases and familiar history of thyroid diseases were similar in the three groups. TSH was significantly higher in obese women (1.8 (IQR 1.4-2.4) mU/L) when compared to normal weight (1.6 (IQR 1.2-2.2) mU/L) and overweight (median 1.6 (IQR 1.2-2.2) mU/L) ones (p<0.001). BMI was significantly related with the risk of having a TSH≥4 mU/L at logistic regression, independently from non-thyroid autoimmunity, smoking or familiar predisposition for thyroid diseases (OR 1.125, 95%CI 1.080-1.172, p<0.001). A restricted cubic splines regression showed a non-linear relationship between BMI and TSH. Women with a TSH≥4 mU/L had a higher UtA-PI, independently from BMI.

CONCLUSIONS: overweight/obesity is significantly related with TSH serum levels in AbTPO negative pregnant women, independently from the other risk factors for hypothyroidism during pregnancy. The increase of TSH levels could be clinically relevant, as suggested by its association with abnormal UtA-PI, a surrogate marker of abnormal placentation.