Understanding Feline Hyperthyroidism

in simple terms

I wanted to keep this post simple and short, but know that it’s a much needed post on an area cat guardians need advice on.

I also wanted to write about this subject matter coming from the perspective of a cat guardian who has a cat with Feline Hyperthyroidism.

Meet Zaza. She is 14 years old and was diagnosed with Feline Hyperthyroidism one and half years ago.

This is Zaza through various stages from being healthy to struggling.

understanding Feline Hyperthyroidism
Zaza in good health
understanding Feline Hyperthyroidism
Zaza having lost quite a bit of weight and coat condition going
understanding Feline Hyperthyroidism
Zaza after she had collapsed




Zaza on thyroid drugs but not coping too well. Note how skinny she has become and looking tense.
















understanding Feline Hyperthyroidism
Zaza weighing 4kg with bad coat condition and loss of fur. The loss of fur is from over-grooming after a bad reaction to her first choice of medication. This is Zaza on no drugs as we try to get the itchiness down before starting a new treatment of medication.



Thyroid glands are found in the neck. Your cat has two thyroid glands.Thyroid glands control a cat’s metabolism.


Hyperthyroidism is when the thyroid stops working as it should and starts to produce too much Thyroid hormone, which increases the body’s metabolic rate.

This can add pressure on the heart and means, despite a cat eating lots of food, the metabolic rate within the animal is overworking, thus the cat continues to lose weight. It’s like the body thinks it’s working out and using up vast amounts of energy.

A blood test to find out the levels of T4 the thyroid is producing will determine whether a cat has Hyperthyroidism. A typical healthy T4 level will come out around the numbers of 45. When Zaza was last tested her T4 levels (thyroid hormone levels) were 270!

A cat with Hyperthyroidism will be hungry all of the time, drink more, beg for food more, lose weight despite constantly eating and also show a decline in coat condition. A cat with Hyperthyroidism  may also start to loose fur and over-groom, so it’s important the cat gets a blood test as soon as possible to determine the cause of the symptoms.

Many cats do not show a swelling of the thyroid gland so it’s not so obvious when the vet is doing a physical examination. Some swellings can be cancerous. The majority are not.


The list below shows some of the options cat guardians should consider when thinking of treatment.

    Medication – this can be oral, tablets or a gel on the ear. Th medication option is for life and is usually given 2 times a day. This can be stressful for cats that hate to be given medication but is a good option for cats that are more comfortable with meds.

  • Diet. A low iodine diet can be used to treat Hyperthyroidism. Reducing iodine in your cat’s diet prevents the thyroid being able to over produce thyroid hormone. If you do decide to use this as a treatment option your cat cannot be fed anything else.
  • Surgery. This option is good for younger cats and can be performed if the vet can feel the enlarged thyroid gland, so they know which one to remove. It’s not so much a good option if the cat is elderly or if the gland is not swollen, because the vet won’t know which thyroid is not working.
  • Radioactive iodine treatment. This is said to be the best option but its downside is hospitalisation plus a degree of isolation for the cat, which could be a substantial length of time.

All of these options should be discussed with your vet, so that the best option for your cat can be determined. The majority of cat guardians go down the medication route.

The vast majority of hyperthyroid cats can be treated successfully. However, it’s important to remember:

  • If your cat has one of their thyroid glands removed, the other gland may develop the same problem further down the line. If this happens, symptoms will return.
  • Removing both thyroid glands puts your cat at risk of developing low calcium (1)




    Yes! Most cats live a happy and healthy life with Hyperthyroidism as long as the condition is managed in the correct way. My cat, Zaza, collapsed with the condition because we had no idea she had it until it was really bad. That is when she ended up in an emergency hospital and rushed to a specialist hospital called The Ralph.



    Most cats do not experience any negative issues with medication for Hyperthyroidism. However, some cats can suffer side effects such as itching/skin lesions, making them uncomfortable and prone to licking and over-grooming. This is what happened to Zaza. After getting her thyroid under control with a drug called Felimazole, Zaza was one of the few cats that experienced side effects. She began to furiously itch all over and started to pull her fur out with exasperation. She was taken off Felimazole and is now trying the oral liquid drug called Thyronorm. Luckily, there are various medications options so if one does not ‘agree’ with your cat there are always other options to try. All cats react differently to treatment and the majority of cats do not experience any side effects with the most common drugs for treating Hyperthyroidism. Zaza was just unlucky.



    If your cat’s treatment goes down the surgery or radiation route, then regular blood tests do not need to be taken afterwards. If your cat is on medication, your vet will probably want you to have regular blood tests to ensure the thyroid levels are staying within the normal range.

    It’s important to know that some of the symptoms of Hyperthyroidism are similar to kidney disease, which is why the blood tests are extremely important. Some cats have both diseases and kidney disease is very common in elderly cats.

    I shall post an update once Zaza has been on her new meds for 4 weeks. We’re hopeful that she will feel better on them.

    Hope this brief article has helped you understand Hyperthyroidism.

    (Source 1: PDSA)


    understanding Feline Hyperthyroidism ABOUT THE AUTHOR

    Anita Kelsey holds a first class honours degree in Feline Behaviour and Psychology (work based BA Hons) and runs a vet referral service dedicated strictly to the diagnosis and treatment of behaviour problems in cats. She is also a qualified cat groomer and specialises in grooming aggressive or phobic cats. Anita writes for Your Cat Magazine and is on their experts panel answering readers questions on cat grooming. She also advises on feline behaviour for the CFBA (Canine and Feline Behaviour) magazine as well as being a full member. Anita, a strong advocate of a vegan lifestyle, is based in London but consults all over the UK as well as international requests. She lives with her husband, a music producer, and two Norwegian Forest cats, Kiki and Zaza.

    Her first book ‘Claws. Confessions Of A Professional Cat Groomer‘ was published by John Blake in 2017 with her second book, Let’s Talk About Cats just released on Amazon US and UK.

    understanding Feline Hyperthyroidism