Anti-thyroid medication

Cat hidingAnti-thyroid medication stops the thyroid glands from overproducing thyroid hormones.  They are the most common way of treating hyperthyroidism, at least initially.

There are three licensed medication options for cats in the UK: methimazole (thiamizole) and carbimazole tablets and a methimazole liquid.  Methimazole (also known as thiamizole) is the active drug – carbimazole is converted into methimazole inside the body.  These medications work by preventing the thyroid gland from producing excessive amounts of thyroid hormone.  They can be used initially to stabilise the patient and ensure no other diseases become unmasked when the thyroid hormone level reduces (e.g. kidney problems), and can also be used for long-term management.

Easypill Cat puttyMethimazole tablets or liquid are usually given twice daily, carbimazole tablets come in a slow-release formulation that is given once daily.  The tablets should be administered whole and the liquid should be administered directly to your cat.  Click here for information about how to give tablets to your cat. Medication does not cure hyperthyroidism, it only controls the disease so, unless a different method is chosen, the cat will need life-long treatment.  All cats respond differently to these medications, and regular check ups and blood tests will be required to ensure that the correct dose is being used and to check for side effects.  Generally, the first blood test will be three weeks after starting the medication.



Cat stretchingAs with any drug, anti-thyroid medications do carry the risk of side-effects.  The main ones to look out for are listed below:

  • Itching, particularly of the face
  • Vomiting and diarrhoea
  • Lethargy
  • Reduced appetite
  • Blood in vomit or faeces
  • Abnormalities in white blood cell count

Some of these mild side effects will resolve over time and do not mean that treatment has to be discontinued.  Some of the more serious side effects will mean that medication needs to stop, and an alternative treatment option considered.  These side effects will usually resolve once treatment ceases.

There have been some positive studies into using a methimazole gel, which is applied to the inside of the ear flap and absorbed through the skin.  This treatment is not licensed for use in the UK so should not be used as a first-line treatment, however if your cat is unable to take oral medication and other treatment options are also not appropriate, your vet may be able to prescribe this medication.  Side effects will be the same as those above.

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