We studied the effects of thyroid hormone (T3) on GH, TSH, and T3 receptor (TR) gene expression as well as deiodinase activities during rat pituitary development. By reverse transcriptase-polymerase chain reaction, GH and TSHβ transcripts were detectable on fetal day 15. Although with certain differences, the expression of both GH and TSHβ genes was under T3 control during fetal and neonatal life. Differences in plasma, but not pituitary, TSH concentrations were observed between control and hypothyroid animals throughout the period studied. Both TRα and TRβ genes were expressed in the fetal pituitary. TRα1, TRβ2, and c-erbAα2 transcripts displayed a developmental profile different from that of TRβ1. Thyroid hormone repressed TRα1, TRβ2, and c-erbAα2 and stimulated TRβ1. Type I and type II deiodinase activities (5'DI and 5'DII, respectively) had different ontogenic patterns; 5'D-II was the predominant activity in fetuses, with levels similar to those in adults, whereas the level of 5'D-I was low and increased with age. T3 stimulated 5'D-I and decreases 5'D-II. These results demonstrate that in somatotroph and thyrotroph cells, the mechanisms responsible for T3 action are mature and active very early in development and suggest an involvement of this hormone in the establishment and/or maintenance of the somatotroph and thyrotroph phenotype.Peer Reviewe