Human growth hormone (hGH) is known to play a functional role in regulating hematopoiesis, although its direct effect on thrombopoiesis is unclear. In this study, we show for the first time that hGH has a distinct capacity to promote the differentiation of human primary megakaryocytes derived from umbilical cord blood CD34(+) cells. In particular, hGH is potent in facilitating proplatelet formation and platelet production from cultured megakaryocytes. The stage- and time-specific activations of extracellular signal-regulated kinase 1/2 and protein kinase B signaling pathways are involved in the action of hGH. Fusion with hGH enhances the effect of a tandem dimer of thrombopoietin mimetic peptide (dTMP) on thrombopoiesis, manifested by a significant acceleration and increase of platelet production, indicating that hGH may exert a complementary and synergistic effect with c-Mpl ligands on thrombopoiesis. Accordingly, the administration of dTMP-growth hormone fusion protein led to a rapid platelet recovery in mice with severe thrombocytopenia induced by 6.5 Gy total body irradiation, thereby markedly abridging the duration of thrombocytopenia crisis (platelets <150 × 10(9)/L), in comparison with high doses of dTMP. These findings demonstrate the functional role of growth hormone in promoting thrombopoiesis and provide a promising avenue for the treatment of severe thrombocytopenia.