Also known as thyrotoxicosis, hyperthyroidism (FHT) is an endocrine (hormonal) disorder that is most often caused by the overactivity of the thyroid gland. Located in the neck on either side of the windpipe, the thyroid gland produces two hormones T3 triiodothyronine and T4 thyroxine. Both hormones help control metabolism. Feline hyperthyroidism is the most common disease of the endocrine (hormonal) system. According to the AAFC (American Association of Feline Practitioners, between 1.5 - 11.4% of cats globally have hyperthyroidism. One insurance company, Nationwide Insurance, listed hyperthyroidism as the fifth most common medical condition in 2015.
Hyperthyroidism is most commonly caused benign tumour (called an adenoma) involving one or both of the thyroid glands. Both lobes are affected in up to 70% of cases.
Other less common causes of feline hyperthyroidism include over administration of thyroid hormones when treating hypothyroidism (underactive thyroid gland) or a cancerous tumour, (known as a thyroid adenocarcinoma) of the thyroid gland. Less than 2% of cats with hyperthyroidism have a cancerous tumour.
Many cat owners first notice their cat is losing weight despite his usual or sometimes an increased appetite. Other common symptoms include:
- Increased thirst and urination
- Behavioural changes (nervous/jittery behaviour, aggression, over-grooming and or bald patches, hyperactive, increased vocalisation)
- Rapid heartbeat (tachycardia)
- Rapid, shallow breathing (tachypnea)
- Poor coat condition (see photos)
- Thickening of the claws
- Weakness (occasionally)
- Decreased appetite (occasionally)
- Hypertension (high blood pressure)
|Note the skinny appearance of this cat. Her fur is also very rough/unkempt looking which is common. You may also notice that they feel extremely bony, especially along the spine.|
Your veterinarian will observe your cat's clinical signs, palpitate the thyroid gland. Other diseases such as renal failure and feline diabetes have similar symptoms.
Tests your veterinarian will carry out will include a biochemical profile to determine the overall health of your cat. Cats with hyperthyroidism may also have elevated liver enzymes.
Specific blood tests to detect elevated levels of the hormones T3 and T4 are performed. Some cats with hyperthyroidism may show normal levels of these hormones in their blood test. If this is the case then a T3 suppression test may be performed. This involves taking a blood test to check the levels of T3 and T4, 7 oral doses of the thyroid hormone T3 and a blood test after the hormone was given. In a normal cat, the level of T4 will drop, in a cat with hyperthyroidism the T4 levels will stay the same or increase slightly.
Thyroid imaging may be carried out to evaluate the shape and size of the thyroid gland.
Classification of hyperthyroidism:
In 2016, the AAFP issued updated guidelines for classifying hyperthyrodisim in cats as follows.
Increased levels of the thyroid hormones speed up the cat's metabolism, this is known as hypermetabolic state. This leads to a hyperdynamic cardiovascular state in which the heart beats faster. It eventually causes congestive heart failure and secondary hypertrophic cardiomyopathy.
High blood pressure (hypertension) is another common side effect of hyperthyroidism in cats. Hypertension has a serious effect on several organ functions. Swelling and bleeding into the eyes can lead to blindness, hardening of the arteries, a hardening of the heart muscle, which over time can cause congestive heart failure and increased risk of stroke.
There are three options to treat hyperthyroidism in cats. Each treatment has its pros and cons. Treatment should depend on other medical conditions your cat may have (heart disease, kidney failure), availability of a nuclear medicine facility and cost.
Drugs (Methimazole/Tapazole): The use of medication doesn't cure hyperthyroidism, only controls it. Administered daily for the rest of your cat's life. There may be side effects from the use of drugs including vomiting and lethargy. Short term this is the cheapest option, but in the long run, it can prove costly having to pay for daily medication for your cat. Methimazole can be given either in oral or transdermal (applied to the skin inside the ear) form. Advantages of medication are that treatment is reversible if side effects occur (see precaution at the end of this article). Other side effects may include anorexia and vomiting.
Surgery: Removal of the enlarged thyroid lobe(s), known as thyroidectomy. Pros of this option are that it is a permanent cure. Cons include increased anaesthetic and surgical risks due to the effect the disease may have had on the cat's heart and kidneys. If both glands are removed, there is increased risk of accidentally removing the small parathyroid glands, (which among other things regulates the calcium supply in the body), which can cause hypocalcemia (low blood calcium). If both lobes are removed you will have to give your a daily thyroid supplementation.
Radioactive Iodine Treatment: This involves a single injection of radioactive iodine (radioiodine I-131). This concentrates in and destroys the diseased thyroid tissue while leaving the normal thyroid tissue intact. Pros of this option are that the cat doesn't require anaesthesia, there is no need to give your cat medication for the rest of his life and in the majority of cases, it is a permanent cure. In a few cases, treatment will need to be repeated. No damage is done to the parathyroid glands. Cons are that it is expensive in the short term, and can only be performed at a specialist veterinary centre. After treatment, your cat will be slightly radioactive and will have to stay at your veterinarian's until the radioactive levels drop.
In some cats, hyperthyroidism and kidney disease run concurrently. The hyperthyroidism can act to protect the kidneys. This is because hyperthyroidism produces a hyperdynamic cardiac state (increased blood flow), which increases glomerular blood flow (GBF) and glomerular filtration rate (GFR), improving renal function. Once treatment begins and the hyperthyroidism is brought under control, kidney failure may become worse. If kidney failure is suspected, your veterinarian will recommend using drugs to control the hyperthyroidism and closely monitor kidney function. That way, if the kidneys do begin to deteriorate, medication can be re-evaluated or stopped immediately, opposed to surgery or radioactive iodine treatment which is permanent.
Why is feline hyperthyroidism on the increase?
Hyperthyroidism was first noted in 1979, and since then has become the most common endocrine disorder to affect cats. So why has this happened?
No specific breeds are prone to feline hyperthyroidism, although one research paper noted that Siamese and Himalayan cats are at a slightly lower risk of developing the disease (Kass, P.H. et al, 1999) . It is most often seen in older (8-10 years plus) cats and the disease appears to be slightly more common in female cats. Up to 10% of senior cats will develop hyperthyroidism.
There has been research which points towards a link between the consumption of canned food and hyperthyroidism in cats, especially fish or liver flavoured varieties. (Martin, K.M et al, 2000). BPA (bisphenol A) is a chemical that is used to line pet food cans to stop erosion. It is a known endocrine disrupter, meaning it interferes with hormone systems. BPA is also found in a great number of plastic products.
Polybrominated diphenyl ethers (PBDE's) are flame retardants which are in many household products such as furniture and electronic goods and have been found in larger fish. They are another known endocrine disrupter. In the house, cats are exposed through inhalation of dust containing traces as ingestion during grooming and consuming some foods containing PBDE's. Several studies have found high levels of PBDE's in cats, however, no conclusive evidence shows a direct link between PBDE's and hyperthyroidism in cats. Certainly, there is a growing body of evidence towards the health risks of PBDE's.
Other factors have included lack of iodine supplementation in certain brands of cat food, but different reports suggest over-supplementation of iodine is the cause, exposure to well water, exposure to gas fireplaces, cat litter and frequent carpet cleaning. (Edinboro C.H. et al, 2004 and 2010). In fact, the general opinion is that it is likely a number of contributing factors that lead to hyperthyroidism.
Update December 2016. A study in American Chemical Society's Environmental Science and Technology journal points fish flavoured food being responsible for exposing cats to PBDE's and PCB's. You can read more about this here.
More detailed references to this research will be included at the end of this article.
I'm not sure if there is a foolproof way to prevent hyperthyroidism. Certainly, exposure to tinned cat food, especially fish (which can contain high levels of iodine and PBDE's), and those with pop tops seem to pose a greater risk to cats. It would seem that smaller cans are less likely to be lined with BPA.
Use a natural cat litter without deodorisers.
Avoid plastic food bowls and storage containers for pet food and water.
Limit chemicals in the household environment, switch to natural cleaning products as much as possible.
Install a water filter if possible to remove chemicals.
Only heat up food in glass or ceramic containers, not plastic.
While not a preventative, cats over the age of 9 should be tested for hyperthyroidism during their yearly annual exam.
Martin, K.M. et al (2000) Evaluation of dietary and environmental risk factors for hyperthyroidism in cats.
Kass, P.H. et al (1999) Evaluation of environmental, nutritional, and host factors in cats with hyperthyroidism. J. Vet. Intern. 1999.
Edinboro, C.H. et al (2004) Epidemiologic study of relationships between consumption of commercial canned food and risk of hyperthyroidism in cats. JAVMA 224 (6):879-886.
Last updated 16th August, 2016.