Anaesthesia in the hyperthyroid cat: how to prepare it
14-year-old cat with newly diagnosed hyperthyroidism, a heart rate of 240 bpm and a mild murmur. It needs dental extractions, but they are not urgent.
What is most appropriate before anaesthesia?
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A. Anaesthetise it now, using ketamine to maintain blood pressure
Ketamine is sympathomimetic: it increases heart rate and myocardial oxygen demand in an already overloaded heart, with a risk of arrhythmias.
B. Postpone anaesthesia, first control hyperthyroidism with methimazole, assess kidney function and heart and, if needed, control heart rate with a beta-blocker
Correct. For elective anaesthesia it is best to have T4 controlled first (a few weeks of treatment). This reduces cardiovascular risk and reveals true kidney function, which hyperthyroidism can mask.
C. Atropine in the premedication to prevent bradycardia
This cat is already markedly tachycardic. Anticholinergics further increase heart rate and oxygen demand, so they are avoided unless there is genuine bradycardia.
D. High-rate fluids (10 mL/kg/h) during anaesthesia to protect the kidney
The cat with thyrotoxic heart disease is very sensitive to volume overload. Fluids are given at moderate rates while watching for pulmonary oedema.
lightbulb Key takeaway
If the cat has a murmur or gallop rhythm, perform echocardiography before anaesthesia. Reduce stress (gabapentin before the visit) and avoid drugs that increase heart rate. Measure blood pressure before and during anaesthesia.
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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.