Objectives Elevated Thyroid Stimulating Hormone (TSH) concentrations have been associated with adverse pregnancy outcomes. We hypothesise that TSH could have a direct impact on uteroplacental development. We, therefore, sought to describe the ontogeny of TSH receptor (TSHR) in the human placenta and decidua.

Methods Human placenta (n= 90, 7-42 weeks gestation) and decidua (n=36, 7-20 weeks gestation) were obtained. Total RNA was reversed transcribed for quantification of mRNA encoding TSHR by TaqMan RT-PCR using 18S as an internal reference. Western immunoblotting and immunohistochemistry was performed to determine protein expression and localisation within placental tissue.

Results Overall, mRNA encoding TSHR was detectable in 66% of placental samples and 42% of decidual samples. Expression of TSHR mRNA was found in 83% of term placentae compared with only 53% of first trimester placentae. There was increasing placental TSHR mRNA expression with advancing gestation (ANOVA p<0.001). Term placentae expressed 13 fold more TSHR mRNA compared with first and second trimester placentae (both p<0.001). Early third trimester (26-34 weeks) placenta expressed 3 fold more TSHR mRNA compared with first trimester placentae (p=0.014). There was no significant difference in TSHR mRNA expression between first and second trimester placental or decidual samples. Western immunoblotting confirmed TSHR protein expression in first and third trimester placental homogenates. Immunohistochemistry localised TSHR to villous trophoblast.

Conclusion Our study is the first to describe TSHR expression and localisation in human placenta and decidua. The physiological significance of uteroplacental TSHR expression remains unclear, but TSHR may have an increasing role as gestation progresses.