Inhibitor of DNA binding (Id) proteins play important roles in regulating cardiac development via paracrine signaling. Id1/Id3 knockout mice die at mid-gestation with multiple cardiac defects. Single Id knockout studies have not reported cardiomyopathies. To bypass embryonic lethality we used Tie2CRE-mediated recombination to conditionally delete Id1 against global Id3 ablation (Id cDKOs), which develops adult-onset dilated cardiomyopathy. We confirm upregulation of thrombospondin-1 (TSP1) in Id cDKO hearts. Colocalization studies reveal increased TSP1 expression in the vicinity of endothelial cells and near regions of endocardial fibrosis/disruption. Downstream fibrotic molecules were upregulated. Endocardial capillary density was reduced with evidence of vascular distention. Treatment of Id cDKO cardiac explants with LSKL, a peptide antagonist of TSP1 activation of TGFβ, reversed the increased expression of fibrotic molecules. We conducted bone marrow transplant experiments in which we transferred bone marrow cells from Id cDKO mice into lethally irradiated WT mice. The majority of WT recipients of Id cDKO bone marrow cells phenocopied Id cDKO cardiac fibrosis 4 months post-transplantation. Injection of LSKL into adult Id cDKO mice led to downregulation of fibrotic molecules. The results prompt caution when bone marrow transfers from individuals potentially carrying mutations in the Id axis are applied in clinical settings.