AbstractTo understand the effect of fetal thyroid gland disruption on development in swine, we evaluated thyroid hormone levels, growth and developmental characteristics, and gene expression associated with thyroid hormone metabolism in late gestation fetuses exposed to methimazole (MMI). Pregnant gilts were given either oral MMI or equivalent sham from gestation day 85–106 (n= 4/group), followed by intensive phenotyping of all fetuses (n= 120). Samples of liver (LVR), kidney (KID), fetal placenta (PLC), and the corresponding maternal endometrium (END) were collected from a subset of fetuses (n= 32). Fetuses exposed to MMIin uterowere confirmed hypothyroid, with a significant increase in thyroid gland size, goitrous thyroid histology, and dramatically suppressed thyroid hormone in serum. In dams, no differences in temporal measurements of average daily gain, thyroid hormone, or rectal temperatures relative to controls suggests that MMI had little effect on maternal physiology. However, fetuses from MMI-treated gilts exhibited significant increases in body mass, girth, and vital organ weights, but no differences in crown-rump length or bone measurements suggesting non-allometric growth. The PLC and END showed a compensatory decrease in expression of inactivating deiodinase (DIO3). Similar compensatory gene expression was observed in fetal KID and LVR with a downregulation of all deiodinases (DIO1, DIO2, DIO3). Minor alterations in the expression of thyroid hormone transporters (SLC16A2 and SLC16A10) were observed in PLC, KID, and LVR. Collectively, MMI crosses the PLC of the late gestation pig, resulting in congenital hypothyroidism, alterations in fetal growth, and compensatory responses within the maternal fetal interface.