We have previously reported that E pluripoietins are produced in mice after a 
single 20-mg injection of cytosine arabinoside (Ara-C) and that they are able to 
initiate the determination of hemopoietic pluripotent stem cells (CFU-S) toward 
the erythrocytic lineage. However, the mechanism of E pluripoietin release is 
still unclear. Since the stimulating effect of thyroid hormone on erythropoiesis 
is well known, we postulated a link between this hormone and the E 
pluripoietins. In previous papers we demonstrated that L-triiodothyronine (LT3) 
exhibits the capacity of inducing CFU-S differentiation toward erythropoiesis in 
vitro. Two series of data presented here suggest that LT3 acts indirectly on 
CFU-S determination by promoting the release of E pluripoietin-like factors. 
First, the Ara-C injection which induces the production of E pluripoietins in 
mice also promotes an increase in the LT3 plasma level. Second, medium 
conditioned with bone marrow cells exposed in vitro for 90 min to LT3 (even 
though this medium does not contain LT3) has E pluripoietin-like effects, 
inducing CFU-S differentiation toward the erythrocytic lineage.