If you’re the owner of an elderly cat, you’ve probably heard of hyperthyroidism, or an overactive thyroid gland. But for all you dog owners out there, you may be wondering if your canine companion can get thyroid problems like cats do? Well stick around as we take a dive into the thyroid and answer this very question.

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What is the Thyroid?

The thyroid is a small gland located just below the larynx (voicebox or Adam’s apple) in cats, dogs and people alike. It is part of the endocrine system, meaning it produces hormones. Hormones are chemical messengers that are released into the body to cause an effect, such as turning on production of a chemical. The thyroid produces ‘thyroid hormones’. It also produces a hormone called calcitonin that acts to lower blood calcium if it gets too high.

‘Thyroid Hormones’ 

The thyroid hormones are triiodothyronine (T3) and thyroxine (T4). These hormones are metabolism hormones. Both thyroid hormones are released when the thyroid receives chemical messengers from the brain. When released, T3 and T4 increase the body’s metabolism by increasing glucose uptake and usage, and increasing fat breakdown. They also have effects on appetite, heart rate, heat production and oxygen consumption.

Think of these hormones being the accelerator in the engines of our bodies – release thyroid hormones and the engine starts to rev, providing power to our bodies. Use the accelerator sensibly, and the engine will do many thousands of miles. In the long-term, thyroid hormones are also important for development as babies, growth as children/puppies/kittens, and reproduction as adults.

Thyroid Disorders

So we know now what a normal thyroid should do – it is the body’s accelerator controlling metabolism, and also works to keep calcium from getting too high. What happens then if the thyroid starts to malfunction?

Hyperthyroidism and Cats

Hyperthyroidism, or an overactive thyroid, is a common problem in elderly cats over 10 years old. The thyroid tends to get larger over time, but in cats particularly it gets so large it starts producing too many thyroid hormones. We term this a benign growth of the thyroid, to differentiate from rarer cancer of the thyroid.

Cats can sometimes get primary or secondary cancers of the thyroid gland that cause similar problems, but they are much rarer. The exact reason for why cat’s thyroid glands enlarge is unknown, but may be linked to diet, the environment, breed, drugs or even cat litter. We aren’t sure.

Regardless, an overactive thyroid gland is the equivalent of pushing the accelerator straight to the floor 

The engine will start to work very hard, which is bad for the engine. Here, the cat’s metabolism goes into overdrive. The metabolism causes a huge increase in hunger but also increases breakdown of muscle and fat, meaning no matter how much the cat eats it gets skinnier and skinnier. Cats with severe hyperthyroidism are often just skin and bones!

Hyperthyroidism increases blood pressure too which can damage the heart and kidneys, causing heart disease and excessive urination (and excessive drinking to compensate). All the extra waste being broken down from fat and muscle can make cats vomit and have diarrhoea too. On top of all this their hair and skin quality gets poor, and they become very stressed and restless, sometimes aggressive.

The hyperthyroid cat’s ‘engine’ is working on overdrive and starting to fall apart. If untreated, hyperthyroidism itself does not kill but the lack of muscling, heart disease, high blood pressure and kidney disease will.

Luckily, if caught early hyperthyroidism can be treated with drugs, surgery, radiation, or diet 

These will reduce the production of T3 and T4. Cats can often live for years with well-managed hyperthyroidism. We can even cure hyperthyroidism in some cases with surgical or chemical removal of the thyroid gland. 

Hyperthyroidism and Dogs

So back to our dog owners – can dogs get hyperthyroidism? Yes, but very rarely. We said before that cats can rarely get hyperthyroidism due to cancer of the thyroid gland. The same can happen to dogs as well, with similar signs to in cats. Unfortunately, cancers of the thyroid gland are harder to treat and generally involve surgical removal of the tumour. Often the cancer has spread from other areas already and the dog is quite unwell. We are glad that these kinds of tumours are rare.

Hypothyroidism and Dogs

Okay, so the thyroid can become overactive. What about underactive? Definitely. This is known as hypothyroidism and is more common in dogs. If we are sticking to car analogies, think of it like trying to pull off without any revs – your engine doesn’t have enough power to function properly. Dogs with hypothyroidism have slow metabolisms that means their bodies do not have enough energy to function.

There are three kinds of hypothyroidism. The first and more common is disease within the thyroid gland (primary) causing a reduced production of T3 and T4. It can be linked to inflammation, the immune system, cancer, congenital defects or certain drugs. The other two types of hypothyroidism (secondary and tertiary) are diseases of the parts of the brain (the pituitary gland and hypothalamus) that release chemicals controlling the thyroid gland. These are quite rare in dogs.

Dogs with hypothyroidism are generally middle-aged. They appear quite slow, lethargic and often obese. We say they have a ‘tragic look’ in their face (due to droopy skin around the muzzle and eyes, called “myxoedema”). These dogs are quite cold intolerant and will seek out warm places more frequently.

Hypothyroidism also affects the skin, causing hair loss, a thick greasy skin, blackheads and secondary infections. Other rarer signs include neurological and eye issues. It can be quite difficult to diagnose hypothyroidism due to the vagueness of the symptoms. As with hyperthyroidism, hypothyroidism can be treated with lifelong medication to replace the thyroid hormones, and dieting.

Hypothyroidism in Cats 

Acquired hypothyroidism is incredibly rare in cats, mostly occurring due to cancer of the thyroid. However, some cats on treatment for hyperthyroidism can become hypothyroid if they are given too much anti-thyroid medication. This is uncommon and unlikely to happen with a well-managed hyperthyroid cat, but is a possibility if the cat was overdosed accidentally.

Feline hypothyroidism can also occur after surgical or chemical removal of the thyroid gland to treat hyperthyroidism – because the thyroid gland is no longer producing thyroid hormones the cat can start to become hypothyroid. In these cases, we can treat the cat in a similar way to hypothyroid dogs. Cats with hypothyroidism present with similar symptoms to dogs.

In Summary:

Hyperthyroidism is increased metabolism causing the animal to break down its own muscles and secondarily damage its own heart, kidneys and other organs. Their engine is working their body too hard. It is common in cats over 10 but very rare in dogs.

Hypothyroidism is a reduced metabolism causing the animal to not have enough energy to function. Their engine is not providing enough power to their body so they slow down, gain weight and get bad skin. It is found mostly in dogs around 7 years old, and is incredibly rare in cats except as a consequence of treatment for hyperthyroidism.

We hope this little guide has explained a little more about what the thyroid gland is for and how hyperthyroidism and hypothyroidism affects dogs and cats.

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