"Our results demonstrate an evolutionarily conserved function for thyroid hormone in regulating heart cell proliferation and suggest that loss of regenerative potential was a trade-off that allowed mammals to become warm-blooded," Huang said. "For early mammals, endothermy was more advantageous than retention of regenerative potential. But now, with medical improvements allowing us to live much longer, this loss of cardiac regeneration becomes more problematic and is a fundamental cause of heart disease."