Hyperthyroidism in dogs

Hyperthyroidism happens when the body produces too much thyroid hormone. It’s a rare disease in dogs and usually results from cancer of the thyroid gland. Hyperthyroidism is most likely to be seen in older dogs of either sex from around 9-11 years. It’s more likely in larger breeds 

Hyperthyroidism happens when the thyroid glands become overactive. Hyperthyroidism in dogs differs significantly from cats.  Over 98% of hyperthyroid dogs have a rare cancer of the thyroid gland, called thyroid carcinoma. Thyroid carcinomas make up only 2-3 % of all tumours diagnosed in dogs. Treatment options are limited, particularly if the cancer has spread when the condition is diagnosed. Early identification of hyperthyroidism gives the best chance of a longer survival time.



What is hyperthyroidism in dogs?

Hyperthyroidism means there’s too much of the thyroid hormone, thyroxine, in your dog’s system. Thyroxine controls metabolic rate or how quickly the body burns up energy.

Your dog has 2 small thyroid glands in their neck. Most hyperthyroidism cases happen when a tumour develops on a thyroid gland. This growth is usually cancerous, meaning that it will spread to other parts of the body, especially the lungs. The cancer may also produce extra thyroid hormone. And thyroid hormone has profound effects on your dog’s metabolism. Think of metabolism as a car’s acceleration. Hyperthyroidism is like having the accelerator pedal pressed down and stuck to the floor! Everything ‘speeds up’ uncontrollably and the engine burns through much more fuel (food and oxygen). Eventually, various ‘engine components’ (organs) will become damaged and stop working.  So the ‘engine’ starts to fail.

Inside your dog’s body, this means:

  • Increasing nutritional (food) requirements to fuel increased metabolism
  • Increasing heart rate and thickening heart muscle
  • Increasing blood pressure
  • Damage to kidneys and intestine

Thyroid carcinoma is the single most common cause of hyperthyroidism in dogs.

Another potential cause is overdosing with thyroid supplements for dogs with underactive thyroid glands.



Symptoms of hyperthyroidism in dogs

Symptoms of hyperthyroidism relate to increased metabolism, including the increased demand for fuel and the damage to vital organs.

  • Always hungry but losing weight
  • Vomiting and diarrhoea
  • Change in bark
  • Drinking more and peeing more
  • Becoming hyperactive, restless
  • Developing a dull, staring coat
  • Increasing heart rate
  • Breathing difficulties



Are some dogs at higher risk of hyperthyroidism?

Some larger breed dogs may be more at risk of hyperthyroidism, including:

  • Golden Retrievers
  • Siberian Huskies
  • Boxers
  • Beagles

Males and female dogs are equally at risk. This differs to humans, where more thyroid carcinomas happen in women than men.

Are my family or other pets at risk of hyperthyroidism?

Hyperthyroidism affects individual dogs. Your family and other pets are not at risk of getting hyperthyroidism.

However, medicines that suppress your dog’s thyroid hormone levels can have the same effect on humans.

  • Medicine for hyperthyroidism must be handled carefully, especially by women of childbearing age
  • Wash your hands carefully after handling tablets
  • Clean surfaces thoroughly after any contact
  • Keep all medicines out of sight and reach of children and pets.



How do vets diagnose hyperthyroidism in dogs?

Vets diagnose hyperthyroidism based on the following:

  • The ‘history’ – any changes in your dog you report, including losing weight, hyperactivity, vomiting.
  • Physical examination- the tumour may be large enough to feel in your dog’s neck
  • Blood tests- general check and thyroid hormone levels

Further tests to find out the type of tumour and whether it has spread to other parts of the body (metastasised).

  • Biopsy of some lymph glands close to the thyroid gland.  A sample will be sent to a laboratory for testing. –
  • Imaging to find out if and where the cancer has spread.- x-rays, CT Scan, Ultrasound,  MRI.


Vet treatment

How do vets treat hyperthyroidism in dogs?

Best outcomes for thyroid carcinoma are achieved with a combination of surgery and radioactive iodine treatment in dogs without tumour spread.

Preparation for Surgery:

  • Antithyroid medicine to normalise thyroid levels –  tablets (methimazole)
  • Medicine to slow the heart rate and reduce blood pressure – beta blockers


  • Removing the whole tumour where possible (mobile and fairly small tumour) or
  • ‘Debulking’ or removing as much of the tumour as possible
  • Radiation treatment
  • Chemotherapy

Where surgery is not possible or inadvisable, chemotherapy may slow down symptom development

Hyperthyroidism resulting from over-dosing thyroid supplements

Thyroid hormone tablets are given to dogs with an underactive thyroid gland (hypothyroidism). If the dosage is too high, treatment can be gradually reduced to restore balance.

Palliative care:

Surgery and chemotherapy may not be the best option for you and your dog.

Vets can help to limit the effects of hyperthyroidism with:

  • Antithyroid medicine – methimazole
  • A special low-iodine diet (Hills Y/D) to deprive the thyroid gland of iodine, which is essential to make thyroxine.

However, these medicines don’t influence the growth of the cancer itself. Symptoms due to an enlarging mass will include:

  • Difficulty eating
  • Difficulty breathing


Home treatment

How to look after a dog with hyperthyroidism at home

There are no known home remedies for hyperthyroidism in dogs. But home support is essential to ensure your dog gets the most benefit from their vet treatment.

  • Give thyroid and other medicine as prescribed at the correct dose and time.
  • Make sure your dog  takes their medication (no spitting it out behind the sofa!)
  • Ask for help or alternatives if you are struggling to give tablets.
  • Unless your dog is on Y/D prescription food, feed a diet that provides lots of calories (energy), enough fat and high-quality protein such as chicken, rabbit or fish.



Can hyperthyroidism be prevented?

Hyperthyroidism in dogs is usually due to cancer which can’t be prevented. The condition is very rare.


When to worry

When to worry about your dog with hyperthyroidism

As the disease progresses, Worrying signs may relate to an overactive thyroid and an enlarging mass, which may spread to other organs and locations in the body,

These include

  • Severe vomiting
  • Anorexia (not eating anything)
  • Kidney failure
  • Weakness and collapse
  • Difficulty breathing
  • Sudden blindness, pacing and head-pressing
  • Seizures

Find your nearest vet if your dog collapses or suddenly seems blind.

Hyperthyroidism in dogs is a rare but very serious illness, usually resulting from cancer. It’s likely that their health and quality of life may deteriorate very quickly.

Eventually, euthanasia may be the kindest option. This is the time to give your dog a peaceful and dignified escape with an overdose of anaesthetic.

When to say goodbye?

<QOL >

Joii can help with:

  • Recognising symptoms of hyperthyroidism
  • Choosing and keeping to the right diet for your dog
  • Giving medication
  • Choosing the treatment option best for you and your dog
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