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HYPERTHYROIDISM IN CATS by Pauline Dewberry
When Billy started being sick on a regular daily basis, and he started wailing at all hours of the day and night, I did wonder if he had the onset of feline dementia. But he had other worrying symptoms which made me think it might perhaps be hyperthyroidism. He has always drunk a lot of water, but his appetite suddenly went from healthy to abnormal – most of the time my three cats eat half a pouch of wet cat food at each meal, rarely will they ask for any more between their meal times. So when Billy polished off his breakfast with alacrity and then sat there looking up at me asking for more like Oliver Twist, I was surprised when he not only ate the second portion but then went to Sam and Ollie’s dishes and finished up what they’d left.
You’d think with all this ‘binge-eating’ he’d be the size of a jungle lion but no, Billy was visibly diminishing before my very eyes. He weighed next to nothing and that set alarm bells ringing. I’d seen an article in a cat magazine a year or so back and I wondered if he might have hyperthyroidism. When he saw the vet, his symptoms didn’t add up at first as our vet couldn’t find the goitre which is usually situated on the thyroid gland and Billy’s heart rate was normal for a cat at the vet’s. Blood tests confirmed that Billy did, in fact, have hyperthyroidism despite the lack of two of the deciding factors being present and he now only weighs 3 kilos (about 6 pounds). So what is feline hyperthyroidism? Hyperthyroidism occurs when a cat’s thyroid glands, located in his neck, start producing too much thyroid hormone. In most cases, this is due to a benign (non-cancerous) change. Hyperthyroidism appears mostly in middle- to old-aged cats and Billy, at almost 15 was a prime target. ‘Classic’ signs of the condition are weight loss despite an increased appetite, increased thirst, increased irritability and restlessness. Many hyperthyroid cats have a rapid heart rate and an unkempt coat. Mild diarrhoea and/or vomiting are also common. Secondary complications: Thyroid hormones have effects on virtually all the organs in the body and therefore it isn’t surprising that this disease can sometimes cause secondary problems that may lead to the necessity for additional investigations and treatment. The effect of thyroid hormones on the heart is to stimulate a faster heart rate (more rapid beating of the heart) and a stronger contraction of the heart muscle. Over time, with hyperthyroidism, the muscle of the largest chamber of the heart (the left ventricle) enlarges and thickens – so called ‘left ventricular hypertrophy’. If left untreated and unmanaged, these changes will eventually compromise the normal function of the heart and can even result in heart failure. However, once the underlying hyperthyroidism has been controlled, the cardiac changes will often improve, or may even resolve completely. Hypertension (high blood pressure) is another potential complication of hyperthyroidism and can cause additional damage to several organs including the eyes, kidneys, heart and brain. If hypertension is diagnosed along with hyperthyroidism, drugs will be needed to control the blood pressure to reduce the risk of damaging other organs. As with heart disease, following successful treatment of the hyperthyroidism, the high blood pressure will sometimes resolve and permanent therapy may not, therefore, be necessary. Kidney disease (chronic renal failure) does not occur as a direct result of hyperthyroidism, but the two diseases often occur together because they are both common in older cats. Care is needed where both these conditions are present as the hyperthyroidism tends to increase the blood supply to the kidneys, which may improve their function. Blood tests taken to assess kidney function in a hyperthyroid cat may show normal or only mild changes, but potentially more severe renal failure may be masked by the presence of the hyperthyroidism. Reaching a diagnosis: On examination, one or two enlarged thyroid glands can often be felt as a small, firm mass in the neck (these are often the size of a pea or a baked bean in hyperthyroid cats). However, in some cats there is no palpable thyroid enlargement, and this can be because the overactive tissue is present in an unusual (ectopic) site (often within the chest cavity). The diagnosis is confirmed by determination of thyroid hormones in the blood. A blood test looking at thyroxine (T4) concentration is usually all that is required for the diagnosis as this is usually elevated in clinical cases. Treatment: There are three main options for the treatment of hyperthyroidism: each with advantages and disadvantages: Medical management (drug therapy): Anti-thyroid drugs are available in tablet form and these act by reducing the production and release of thyroid hormone from the thyroid gland. They do not provide a cure for the condition, but they do allow either short-term or long-term control of hyperthyroidism. · Methimazole (Felimazole; Dechra) a twice daily medication, although once stabilised some cats only require once daily. · Carbimazole (Vidalta; Intervet Schering-Plough) a slow release formulation meaning that it only needs to be given once daily. The tablets must not be broken or crushed. Thyroid hormone concentrations usually fall to within the reference range within three weeks. Treatment is then adjusted according to response. To maintain control of hyperthyroidism, treatment needs to be given daily for the rest of the cat’s life. Surgical thyroidectomy: Surgical removal of the affected thyroid tissue (thyroidectomy) can produce a permanent cure and is a common treatment for many hyperthyroid cats. In general, this is a very successful procedure and is likely to produce a long-term cure or permanent cure in most cats. However, surgery will not be successful if ‘ectopic’ thyroid tissue is present and even after successful surgery, occasionally signs of hyperthyroidism develop again at a later time if previously unaffected thyroid tissue becomes diseased. Radioactive iodine therapy: Radioactive iodine is a very safe and effective cure for hyperthyroidism wherever the location of the overactive thyroid tissue. It has the advantage of being curative in most cases with no ongoing treatment required. It is administered as a single injection given under the skin – the iodine is then taken up by the active (abnormal) thyroid tissue, but not by any other body tissues, resulting in a selective local accumulation of radioactive material in the abnormal tissues. The radiation destroys the affected abnormal thyroid tissue, but does not damage the surrounding tissues or the parathyroid glands. The advantages of radioactive iodine are that it is curative, has no serious side-effects, does not require an anaesthetic and is effective in treating all affected thyroid tissue at one time, regardless of the location of the tissue. However, it does involve the handling and injection of a radioactive substance. This carries no significant risk for the patient, but precautionary protective measures are required for people who come into close contact with the cat. For this reason, the treatment can only be carried out in certain specially licensed facilities and a treated cat has to remain hospitalised until the radiation level has fallen to within acceptable limits. This usually means that the cat must be hospitalised for between three and six weeks (depending on the facility) following treatment. Most treated cats have normal thyroid hormone concentrations restored within 3 weeks of the treatment, although in some, it can take a little longer. For more information, see the Feline Advisory Bureau’s website at www.fabcats.org or speak to your vet. My grateful thanks to the FAB (Feline Advisory Bureau) where I extracted a lot of this material for this article.
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