What is Hyperthroidism?

Hyperthyroidism (overactive thyroid glands) is a very common disorder of older cats.  It is caused by an increase in production of thyroid hormones from the thyroid glands, which are situated in the neck. Thyroid hormones are responsible for regulating many body processes and when too much hormone is produced the clinical signs can be quite dramatic, and cats can become seriously ill.

Thyroid hormones also help control the body’s metabolic rate and cats with hyperthyroidism tend to burn up energy too rapidly and typically suffer weight loss despite having an increased appetite and increased food intake.  Fortunately, the vast majority of cats that develop hyperthyroidism can be treated very successfully and most cats will make a complete recovery.

Causes of Hyperthyroidism

In the vast majority of cases hyperthyroidism in cats is caused by a benign (non-cancerous) change. Cats have two thyroid glands, and in most cases (more than 70%) both are involved in the disease, and become enlarged (a change called ‘nodular hyperplasia’, which resembles a very benign tumour).  The underlying cause of this change is currently unknown, but is very similar to one of the two main causes of hyperthyroidism in humans called ‘toxic nodular goitre’.

Rarely (in less than 1-2% of cases) a malignant (cancerous) tumour (thyroid adenocarcinoma) can be the underlying cause of the disease.

Signs of Hyperthryroidism

Hyperthyroidism is usually seen in middle-aged and older cats, rarely being seen in cats less than 7 years of age.  Male and female cats are affected equally.

In affected cats, a wide variety of signs usually develop, but these are usually subtle at first, and become more severe over time as the underlying disease gets worse.  Also, as it is mostly older cats that are affected, some cats will have other diseases that can complicate and even mask some of the clinical signs.

The ‘classic’ signs of hyperthyroidism are:

• Weight loss
• Usually a good or increased appetite (polyphagia)
• Increased thirst (polydipsia)
• Increased activity, restlessness or irritability
• An increased heart rate (tachycardia)
• A poor and unkempt hair coat

Treatment

Successfully treated cats, irrespective of what treatment is used, will usually have complete reversal of all the signs of hyperthyroidism.  Further evaluation will usually be needed to check kidney function and ensure long-term goals of thyroid hormone concentrations are being achieved, but most cats do remarkably well when treatment is administered.