TPMT enzyme stands for Thiopurine methyltransferase or thiopurine S-methyltransferase. It is an enzyme that metabolizes thiopurine drugs.
Patients with the lowest level of the tpmt enzyme has the greatest number of side effects when exposed to a drug containing a thiopurine because it can't totally metabolize the thiopurine drug and there are more excess thiopurine in the body than normal.