Transcription factor involved in the specification of the lactotrope, somatotrope, and thyrotrope phenotypes in the developing anterior pituitary. Specifically binds to the consensus sequence 5'- TAAAT-3'. Activates growth hormone and prolactin genes (PubMed:22010633, PubMed:26612202). {ECO:0000269|PubMed:22010633, ECO:0000269|PubMed:26612202}. This is the function of POU1F1 (POU class 1 homeobox 1, ENSG00000064835).