Azathioprine in the dog: hepatotoxicity and monitoring
5-year-old German Shepherd with immune-mediated haemolytic anaemia. In addition to prednisolone, you plan to start azathioprine as a second immunosuppressant.
Which statement about azathioprine is correct?
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A. It can be used in the same way in cats
Cats have very little activity of the enzyme that metabolises it (TPMT) and suffer severe myelosuppression. It is not recommended in this species.
B. Hepatotoxicity appears after years of use: an annual check is enough
It usually appears early, in the first weeks of treatment. An annual check would come too late.
C. Alkaline phosphatase is the marker of azathioprine hepatotoxicity
Prednisolone increases ALP through induction, so it is useless for detecting the toxicity. The useful marker is ALT.
D. Hepatotoxicity usually appears in the first weeks (more often in German Shepherds): check ALT and a complete blood count every 1–2 weeks at first
Correct. If ALT rises markedly, stop azathioprine and switch to another immunosuppressant (e.g. mycophenolate). Myelosuppression can appear later, so continue periodic blood counts.
lightbulb Key takeaway
Before starting an immunosuppressant, run baseline bloods. With azathioprine, check frequently in the first month and less often afterwards. If ALT rises, consider other causes too, but do not attribute it to corticosteroids without checking.
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Updated: October 31, 2026 · Educational content for veterinary professionals. It does not replace current clinical guidelines or the judgement of the attending veterinarian.