Hyperthyroidism is the most common hormone disease of cats. It’s caused by and over-active thyroid gland and affects cats over the age of 8. It’s rare in other species so the focus here will be on cats. It’s important to catch this condition early as if treatment is started early, it’s usually successful. If left it can cause serious, life-threatening illness.
What is hyperthyroidism?
Cats have two thyroid glands in their neck which produce the hormone thyroxine. Thyroxine regulates many body processes including the metabolic rate. Hyperthyroidism occurs when these glands enlarge producing excess thyroxine. This enlargement is usually a benign (non-cancerous) growth called hyperplasia. In 70% of cases both glands are eventually affected. Only in 1-2% of cases is the growth cancerous, and the outlook much poorer. When hyperthyroid disease occurs in dogs, the growth is sadly more likely to be cancerous.
What signs might I notice?
The excess hormone increases the metabolic rate producing a classic picture of an aged cat that is losing weight but eating more. Increased appetite is often seen by owners as a good sign, but it’s just as important a sign as eating less and should not be ignored. Cats may be spritelier due to this boosted metabolism, compounding the image that all is well. Cats may be restlessness, irritable, and louder, demanding more food. Signs can initially be subtle, easy to miss, and may be complicated by other age-related issues.
Some cats suffer vomiting, diarrhoea, poor coat, and increased thirst, and may even have a decreased appetite and be weak or lethargic, but it’s not common.
In people, high blood pressure can cause severe headaches. It’s thought cats may suffer similar pain, which may be an additional cause for meowing.
I think my cat may have this, what now?
If this sounds familiar, book a vet appointment as soon as possible. It’s a manageable condition, but if left, more issues can arise.
The increased metabolism causes an increased heart rate. Over time this can lead to thickening of the heart muscle and eventually heart failure.
Around a quarter of cats with hyperthyroidism also have high blood pressure (hypertension). This can damage organs such as the eyes, kidneys, and the brain. The longer without treatment the more damage occurs.
What can my vet do?
The signs alone would cause suspicion. Your vet may feel an enlargement of the thyroid gland in the neck, known as a goitre. Sometimes extra tissue around the glands causes the problem, so a goitre is not always there. A fast heart rate could raise suspicion as would a heart murmur, which may be present if heart disease had progressed. Diagnosis is made using a blood test to measure thyroxine levels (T4) which in 90% of hyperthyroid cats are increased. Occasionally an additional blood test may be needed, measuring thyroxine in a different way. Your vet may carry out blood and urine tests to check for other diseases and measure blood pressure. If heart disease suspected they may suggest other investigations as well.
What is the treatment?
The most commonly used treatments are life-long medication or radioactive iodine, but surgery is also available. Each have different risks, benefits, and costs. Your vet can help you decide which one is best for your cat.
Anti-thyroid medication is the first line treatment. It can get thyroxine levels under control in the short-term while preparing for another option, or as a long-term treatment itself.
Medication comes in tablet form and must be given daily, sometimes twice daily to be effective. This may be challenging for cats that find being medicated stressful. The dose will need adjusting depending on response and T4 levels, checked intermittently. Increasingly, a gel which is applied to your cat’s skin (e.g. on the ears) is being used as an alternative to daily tablets – much to the relief of those of us who own more feisty cats!
Mild side effects such as gastric irritation might occur which usually resolve quickly. More serious problems such as bone marrow suppression, liver changes, and skin irritation are rare and mean stopping treatment.
Radioactive iodine is a safe and effective treatment, curing 95% of cats with just one course of treatment. The iodine is taken up only by over-active tissue, such as within the abnormal gland. Abnormal tissue is destroyed, leaving normal tissue intact. This option is great but can be costly, and as it involves radiation, can only be done at limited locations able to handle radioactive material. As your cat would become temporarily radioactive, this option would require them to be hospitalised for a short period. Rarer cancerous forms of thyroid growth can be treated with a higher strength version of this.
Surgery is an option if there is a goitre (swelling), but only after 2-3 weeks minimum on medications to stabilise thyroid levels. Removing both glands risks damaging the parathyroid gland that sits alongside the thyroid gland. These glands control calcium levels, so if damage occurs, short- or long-term calcium supplements might be needed. Factors such as heart disease and underlying issues such as kidney disease need to be looked at when deciding on levels of general anaesthetic risk. On the flip side, cats will usually not need anti-thyroid medication anymore after surgery. If your cat has one of their thyroid glands removed, which is safer than removing two, the other gland may develop the same problem further down the line given that in 70% of cats the problem affects both glands. If this happens, symptoms will return. Surgery is less commonly used here in Australia with many vets preferring either medication or radioactive iodine therapy.
Diets have recently been developed with restricted levels of iodine. Iodine is needed to create thyroid hormones. If a cat is fed ONLY this, it may help to control the level of hormones. This diet is only available from your vet.
Sometimes in treating hyperthyroid disease, hidden kidney disease is uncovered. High blood pressure found in hyperthyroid cats can actually support the kidneys. When treatment takes effect, reducing the blood pressure to normal levels, your cat may start to show signs of kidney disease as that support is taken away. Kidney disease is common generally in older cats so many cats need both conditions managing simultaneously, which can be tricky. Heart disease or hypertension must also be treated and monitored.