Transcription factors (TFs) regulate cell fates, and their expression must be tightly regulated. Autoregulation is assumed to regulate many TFs’ own expression to control cell fates. Here, we manipulate and quantify the (auto)regulation of PU.1, a TF controlling hematopoietic stem and progenitor cells (HSPCs), and correlate it to their future fates. We generate transgenic mice allowing both inducible activation of PU.1 and noninvasive quantification of endogenous PU.1 protein expression. The quantified HSPC PU.1 dynamics show that PU.1 up-regulation occurs as a consequence of hematopoietic differentiation independently of direct fast autoregulation. In contrast, inflammatory signaling induces fast PU.1 up-regulation, which does not require PU.1 expression or its binding to its own autoregulatory enhancer. However, the increased PU.1 levels induced by inflammatory signaling cannot be sustained via autoregulation after removal of the signaling stimulus. We conclude that PU.1 overexpression induces HSC differentiation before PU.1 up-regulation, only later generating cell types with intrinsically higher PU.1.