Hyperthyroidism in Old Cats - Cloud 9 Vets

Hyperthyroidism in Old Cats


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This is a common hormone disorder of middle aged to old cats. It is rarely seen in cats under 8 years old. The average age for a diagnosis is 13 years.

It is caused by a benign tumour of the thyroid gland called an Adenoma.

There is an increased production of thyroid hormones from the enlarged thyroid gland in the neck.

The Thyroid hormones regulate the body’s metabolic rate and affect all tissues and each system of the body.

Too much hormone results in hyperactivity in all body functions.

The changes are subtle and often people put weight loss and poor coat down to old age.


  • Weight loss
  • Increased appetite
  • Restlessness
  • Matted, greasy or poor coat
  • Fast heart rate
  • Increased activity
  • Vomiting
  • Diarrhoea
  • Increased drinking.

These symptoms worsen as the disease progresses if it is left untreated.

Hyperthyroid is confirmed by blood tests that show elevated thyroid levels.

Thyroid hormones affect nearly all organs in the body therefore thyroid disease often causes secondary problems, including the heart and therefore can cause heart disease.

Elevated thyroid hormones stimulate an increased heart rate and blood pressure and strong contractions of the heart muscle and can cause thickening of the left Ventricle over time and compromise the normal function of the heart and lead to heart failure.

High blood pressure

High blood pressure causes damage to several organs like the eyes, kidneys, heart and brain.

There are 3 treatments

1. Food

There is a specific thyroid diet which has only just enough iodine for normal thyroid function. It is a balanced senior diet and the food is gradually introduced over 10 days then fed exclusively from then on.

2. Medication

Anti thyroid drugs which act by reducing the production and release of thyroid hormone.

Regular blood tests to check on the correct dose.

The medication is potentially toxic, bloods are monitored regularly.

These do not provide a cure for the disease and can cause side effects.

1 tablet 2 x a day for he rest of the cats life.

Some owners find it difficult to give medication.

Side effects are vomiting or allergic reactions to the tablets.

3. Surgical

This involves removing one or both of the affected lobes of the thyroid gland under general anaesthetic.


General Anaesthetic risks in an elderly cat.

Damage of the parathyroid glands which are located within the thyroid glands. These are crucial for maintaining stable blood calcium levels.

Damage to local nerves during surgery.

Radioactive Iodine therapy
Radioactive iodine is administered as an injection under the skin and is quickly absorbed into the blood stream concentrates in the thyroid follicles.
The iodine is absorbed and destroys the abnormal thyroid tissue but does not damage the surrounding tissues or parathyroid glands.
It is curative with no side effects. No surgery or general anaesthetic.
Patients will need to be hospitalised for 3 – 6 weeks of no contact.

1% of thyroid masses are cancerous, these will spread to the liver and unfortunately the prognosis for these cats is poor.

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by Cloud 9 Vets on September 24, 2019

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Hyperthyroidism in Old Cats

This is a common hormone disorder of middle aged to old cats. It is rarely seen in cats under 8 years old. The average age for […]