Abstract: The aim of this study was to investigate the expression pattern of DIO3 during the embryonic development in mice, assess the effects of intragastric gavage of levothyroxine (L-T₄) on DIO3 expression and embryo number, and provide basic data for the study of maternal thyroid hormone abnormalities during pregnancy. In this study, the healthy Kunming mice (70 females, 35 males) aged 6 weeks were used as experimental objects. After adaptive feeding for one week, female mice were given intraperitoneal intragastric administration of 10 U PMSG (dissolved in 0.3 mL normal saline). After normal feeding for 48 h, male and female mice were caged together at the ratio of 1∶2 overnight. Once the milky vaginal suppository was visible in the morning of the next day, it was considered as the first day of pregnancy. Pregnant mice were randomly divided into 5 groups. The mice in experimental groups were intragastric administrated with 0.25(SG, n=10), 1.25(MG, n=10), 2.50(LG, n=10) and 5.00 μg·μL^-1(HG, n=10) of L-T₄, re-spectively, and the control group was intragastric administrated with the same amount of normal saline(CG, n=10), 80 μL per day for 10 days. Placentas were collected from mice in exprmental groups on the 10th and 18th day of gestation (n=4/group) and the numbers of embryos were recorded. In addition, ovaries of non-pregnant mice and placentas of mice from the control group on the 10th, 14th and 18th day of gestation (n=3/group) were collected. The expression of DIO3 in placentas of mice was detected by RT-qPCR, in situ hybridization and immunohistochemistry. The contents of TSH, FT₃ and FT₄ in serum of pregnant mice were detected by chemiluminescence meter after intragastric administration of L-T₄. The results of RT-qPCR showed that，under normal physiological condition, the expression level of DIO3 in placenta decreased with the increase of gestation time, and the highest expression level was found on the 10th day of gestation, and the lowest expression level was found on the 18th day of gestation, which was lower than that in non-gestation (P<0.01). In situ hybridization and immunohistochemistry analysis of paraffin sections were consistent with the results of RT-qPCR. The serum levels of TSH, FT₃ and FT₄ increased with the increase of concentration of L-T₄ intragastric administration, and the differences among groups were significant (P<0.05). In addition, the expression of DIO3 increased with the increase of intragastrically administered dosage of L-T₄. On the 10th day of gestation, there was a significant difference in DIO3 expression between the control group and HG group (P<0.01), there was no significant difference between the control group and the SG, MG and LG groups. On the 18th day of gestation, the expression of DIO3 in the control group was significantly higher than that in gavage groups (P<0.01). On the 10th day of gestation, the number of embryos of pregnant mice in the gavage groups was slightly higher than that in the control group. On the 18th day of gestation, the number of embryos of pregnant female mice in the control group and the gavage groups was approximately the same. In conclusion, intragastric administration of L-T₄ could increase the level of thyroid hormone in the serum of pregnant mice and induce the expression of DIO3 in the placenta. However, intragastric administration of L-T₄ had no significant effect on the number of embryos in pregnant mice. This study provides a theoretical basis for further study on the molecular mechanism of thyroid hormones level regulation during pregnancy.

Key words: mice, gestation, DIO3, gene expression, thyroid hormones