Feline Hyperthyroidism: An Evidence Based Approach

Presenter – Andrew L. Bodey BVSc CertVR MRCVS. Andrew established the first radio-iodine unit in the North of England in 2008 and in the process he reduced the minimum hospitalization period to 2 weeks. In September 2013 he founded the Hyperthyroid Cat Centre, near Wetherby, providing radio-iodine treatment on a much larger scale, and further reduced the minimum hospitalization period to 7 days. 

As a pre-Christmas treat, The Webinar Vet organised for Andrew L. Bodey to lead a veterinary webinar discussing the aetiology and management of feline hyperthyroidism using evidence based medicine. In my experience, after twenty years in practice, hyperthyroidism in cats is a disease on the increase, and according to Andrew this is also the case when looking at published data. There is evidence to show that feline hyperthyroidism has had a continually rising prevalence since 1979, with some sources stating up to 20% of cats over the age of 10 years could be suffering from the disease. Evidence also suggests that hyperthyroidism is under diagnosed and could be present in around 10% of well cats.

Due to the high incidence of feline hyperthyroidism , risk factors for the development of the disease have to be considered. According to Peterson 2012, feeding a commercially prepared cat food is one of the leading candidates considered to be a major risk factor. Soy protein is present in a number of cat foods and contains goitrogens which can interfere with thyroid production. Iodine excess and/or deficiency may also play a role through metabolic dysfunction and back in the 1980’s commercial cat food contained ten times the recommended levels of iodine. These levels were reduced in the 1990’s with 25% of foods being below the level of detection.

Other possible risk factors include chemicals such as bisphenol A which lines plastic containers and is very similar in structure to T3 and T4. This chemical is often found in food and drink and could potentially interfere with thyroid metabolism.  Andrew went into further detail about other potential risk factors including flame retardants, pesticides such as flea control and components of cat litter which may also play a role. Practical measures which may help to prevent the development of the disease were also discussed.

I’m sure the majority of us are familiar with treatment options available for hyperthyroidism one of which includes medically managing the disease using oral methimazole. However as we all know, medicating cats on a regular basis can be challenging for owners and Andrew went on to discuss an alternative, transdermal methimazole gel. The ease of application of this product was considered significantly easier than medicating with oral methimazole. Increasing doses of this gel is likely to be required over time but it does allow more accurate dose variation compared to the oral product. Once or twice daily application of this product has also shown to provide T4 stability over 24 hours. Side effects for this gel include the development of sore skin at the site of application.

An iodine restricted diet is another form of treatment recently made available to us. According to trials, 64% of cats were euthyroid at 4 weeks and 75% of cats were euthyroid at 8 weeks after starting the iodine restricted diet. However a number of cats didn’t complete the trial due to the diet’s lack of palatability in some cases. Interestingly there was no difference in results between  indoor and outdoor cats and there was no statistical difference in response to the diet between week 4 and week 8. This probably means that if a cat hasn’t responded well by week 4, the likelihood is there will be no further improvement by week 8.

Andrew went on to discuss other treatments such as radioactive iodine in much greater depth and provided a really interesting and up-to-date account of all the treatment options available to us, and with a disease so frequently diagnosed this really is a veterinary webinar worth investing an hour of your time.

The Stethoscope (MRCVS)


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