Thyroid hormone has pleiotropic regulatory effects on growth, development, and metabolism. It also regulates cell proliferation, differentiation, and apoptosis. In the present study, differentially expressed genes (DEG) and their related pathways involved in TH regulation during gestation were identified in the placenta tissues of high thyroid-induced mice by RNA sequencing. The levels of TSH, FT₃, and FT₄ in blood showed an increasing tendency in the high thyroid-induced mice with intragastric administration of 0.250 or 0.500 µg•µL^-1 L-T₄, respectively. Compared to CG9d mice, one DEG and 10 DEGs were identified in the LG9d and HG9d groups, respectively. There were 32 DEGs identified between the LG9d and HG9d, 22 of which were up-regulated DEGs and 10 were down-regulated. Several GO annotations and signaling pathways were associated with embryonic development and TH synthesis and metabolism, including the Wnt signaling pathway and the TH signaling pathway. The expressions of nine randomly selected DEGs were re-quantified by quantitative real-time PCR (qPCR) and showed a consistent tendency with the results of RNA-sequencing, which suggests that reliable data are obtained in the transcriptome assay. These findings provide insights into the mechanism of the dynamic regulation of the TH balance during pregnancy in mammals.