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.