Lactate dehydrogenase (LDH) is a tetrameric protein consisting of two types of subunits, called M and H, which have small differences in amino acid sequence. Different molecular forms of an enzyme are called isoenzymes or isozymes.

The tissue specificity of isoenzyme patterns of LDH is useful in clinical medicine. Such pathological conditions as myocardial infarction, infectious hepatitis, and muscle diseases involve cell death of affected tissue, with release of cell contents to the blood. The pattern of LDH isoenzymes in the blood serum is representative of the tissue that released the isoenzymes. This information can be used to diagnose such conditions and to monitor the progress of treatment.