Thiopurine S Methyltransferase Deficiency: Causes, Symptoms, and Treatment
Medically reviewed by Harsha Moole, MD, Internal Medicine. Last reviewed: October 2, 2026.
What kind of doctor treats Thiopurine S Methyltransferase Deficiency?
A primary care doctor often orders the first test or spots the problem when a medicine causes side effects. Care usually involves a specialist too, such as a hematologist, a gastroenterologist, or a transplant team. A direct primary care or concierge doctor can be a first stop for an evaluation and can refer you to a specialist when needed.
What it is
Thiopurine S methyltransferase deficiency, often called TPMT deficiency, means the body makes little or none of an enzyme that processes medicines called thiopurines. These medicines treat blood cancers, transplant rejection, and immune diseases like Crohn's disease and lupus. With the deficiency, they can build up and cause serious side effects, including a very low white blood cell count.
Symptoms
Most people with TPMT deficiency feel normal until they take a thiopurine medicine. After starting one, they may develop bone marrow suppression and stop making enough blood cells. Signs include unusual tiredness, frequent infections, easy bruising, bleeding gums, mouth sores, and fevers.
Causes and who gets it
TPMT deficiency is inherited through genes passed from parents to children. Someone with one changed copy of the TPMT gene makes a reduced amount of the enzyme; someone with two changed copies makes very little or none. Roughly one in a few hundred people of European descent has two changed copies.
How doctors diagnose it
Doctors can test for TPMT deficiency with a blood test that measures how well the enzyme works, or a genetic test that looks for changed copies of the gene. Many doctors order testing before prescribing a thiopurine so they can choose a safe plan.
Treatment
No treatment is needed for the deficiency itself. The main goal is to use thiopurine medicines safely. People with low enzyme activity usually need much smaller doses, or a different medicine altogether, and doctors often check blood counts during treatment.
When to see a doctor
Anyone who starts a thiopurine medicine and then feels unusually tired, gets sick often, bruises easily, or has mouth sores and fever should contact a doctor right away. These can be signs of a dangerous drop in blood cells.
How a direct primary care or concierge doctor fits in
A direct primary care doctor charges a flat monthly fee instead of billing insurance, while a concierge doctor usually charges more and often works alongside insurance. Either one can order TPMT testing and review the results with the patient. When a thiopurine is needed, they refer to a specialist and stay involved for follow-up care.
Common questions
Can I live a normal life with TPMT deficiency?
Yes. The condition only matters when thiopurine medicines are involved. With testing and careful dosing, most people can be treated safely with these drugs or with alternatives.
Should my family members get tested?
It can be worth discussing with a doctor, since the condition runs in families. A genetic counselor, a professional trained in inherited conditions, can help explain the options.
Is the test covered before I even take these medicines?
Many insurance plans cover TPMT testing when a thiopurine prescription is planned, but coverage varies. Direct primary care and concierge practices often include such labs in their fee or offer them at lower prices.
What happens if I take a thiopurine and I have the deficiency?
The drug can build up and damage the bone marrow, the soft tissue inside bones that makes blood cells. This can lead to severe infections or bleeding, which is why testing and monitoring matter so much.
Sources
- MedlinePlus Genetics - Thiopurine S-methyltransferase deficiency
- NIH Genetic and Rare Diseases Information Center - Thiopurine S-methyltransferase deficiency
Medical Disclaimer
This content is for general educational and informational purposes only and is not medical advice. It is not a substitute for professional medical advice, diagnosis, or treatment, and reading it does not create a doctor-patient relationship. Always seek the advice of your physician or another qualified health provider with any questions about a medical condition, and never disregard or delay seeking professional advice because of something you read here. If you think you may have a medical emergency, call 911 (or your local emergency number) immediately.
