Lactate dehydrogenase (LDH, or LD) is an enzyme found in almost all body tissues. Usually the concentration in the blood is low, because  it usually stays contained within the tissues’ cells. When cells are damaged or destroyed, however, they release LDH into the bloodstream, causing blood levels to rise. For this reason, LDH is used as a general marker of injury to cells; it is not useful for determining which specific cells are damaged.

Source: LabTestsOnline