Urinary tetracycline antibiotics exposure during pregnancy and maternal thyroid hormone parameters: A repeated measures study

Background: Studies on potential maternal thyrotoxicity related to tetracycline antibiotics exposure during pregnancy are lacking. Based on a large prospective cohort study, this study aimed to examine the associations between tetracycline antibiotics exposure in maternal urine and maternal thyroid hormone parameters. Methods: Based on the Ma'anshan Birth Cohort study, urine and serum samples of 2969 pregnant women were collected in the first, second and third trimesters. Tetracycline antibiotics, including oxytetracycline, chlorotetracycline, tetracycline and doxycycline in urine samples, as well as free thyroxine (FT4), thyroid stimulating hormone (TSH), total triiodothyronine (TT3) and total thyroxine (TT4) levels in serum samples, were measured. Linear mixed models and multivariate linear regression models were employed to examine associations between tetracycline antibiotics exposure during pregnancy and maternal thyroid hormone parameters. Results: The detection rates of four individual tetracycline antibiotics and all antibiotics (sum of four individual tetra-cycline antibiotics) in the three trimesters were 5.0%-52.3%, and the 95th percentile concentration ranged from 0.11 to 4.84 ng/mL. After adjusting for potential confounding factors, the repeated measures analyses indicated that preg-nant women exposed to doxycycline and all antibiotics during the entire pregnancy were negatively associated with serum FT4 and TT4 levels but positively associated with serum TSH and TT3 levels. Trimester-stratified analyses found that doxycycline and all antibiotics exposure during the first trimester were negatively associated with serum FT4 and TT4 levels, while doxycycline was positively associated with TSH levels. In the third trimester, a significant association was only found between all antibiotics and TSH levels.Conclusions: Our results suggest that exposure of pregnant women to tetracycline antibiotics is associated with mater-nal thyroid hormone parameters, and the first trimester might be the most critical window. More studies are needed to substantiate our findings and determine the underlying biological mechanisms.