Feline Hyperthyroidism

Contents

What causes the disease?
Clinical signs of hyperthyroidism
The diagnosis of hyperthyroidism
Treating hyperthyroidism
Felimazole® for the treatment of Hyperthyroidism

What causes the disease?

Hyperthyroidism is the most common endocrine disorder occurring in middle-aged and older cats.  It is caused by the over-production of thyroid hormones from the thyroid gland.

The thyroid gland is located in the neck.  In the cat, the gland is divided into 2 lobes  - one lobe lies on either side of the windpipe (trachea).

The thyroid gland produces 2 hormones:  thyroxine and triiodothyronine, also known as T4 and T3.

Hyperthyroidism occurs when either one or both thyroid lobes enlarge. As a result, the production of thyroid hormones increases and eventually becomes excessive. Exactly why the thyroid lobe(s) start to enlarge is not known.  Fortunately, in over 98% of cases, it is a benign (non-cancerous) change and treatment is very successful. Less than 2% of cases are caused by a malignant (cancerous) growth of the thyroid gland.

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Clinical signs of hyperthyroidism

The thyroid hormones, T4 and T3, regulate the body’s rate of metabolism. Cats with hyperthyroidism have an excessive amount of T4 and T3 in the blood. This dramatically increases their metabolic rate and most consistently results in weight loss, despite the cats also having a ravenous appetite.

Other common signs include:

• Increased restlessness and irritability
• Rapid heart rate
• Unkempt coat
• Diarrhoea
• Occasional vomiting
• Increased urination and drinking

Cats with hyperthyroidism can also develop heart problems and high blood pressure.

Although there is no correlation between the two conditions, hyperthyroidism and kidney disease often occur together as they are both very common in older cats.

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The diagnosis of hyperthyroidism

Diagnosis of feline hyperthyroidism is usually very straightforward. In the majority of cases, a veterinary surgeon will be able to feel the enlarged thyroid lobe(s) in the neck. Sometimes the thyroid gland cannot be felt because the overactive thyroid tissue is located elsewhere, usually inside the chest.

Blood samples taken from a cat to measure the level of T4 (total T4 blood test) will usually confirm the diagnosis.

As most cats with hyperthyroidism are older, routine blood tests to assess organ function are also recommended to make sure that no other conditions will affect the success of treatment.

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Treating hyperthyroidism

Since very few cats with hyperthyroidism have cancerous growths of the thyroid gland, treatment is usually very successful. The treatment’s purpose is to bring the amount of thyroid hormone in the cat’s blood back to normal. This can be achieved in 3 ways:

1. Medical treatment – medication is given every day to reduce the production of thyroid hormones. Medication can be used for short-term or long-term treatment.
2. Surgical treatment – an operation is performed to remove the abnormal thyroid tissue.
3. Radioactive iodine therapy – the abnormal thyroid tissue is destroyed by radioactive treatment. A cat treated in this way needs to go to a special treatment centre and remain there for several weeks.

Most cases of hyperthyroidism are initially treated with daily anti-thyroid medication for 3 weeks to return thyroid hormone levels in the blood back to normal. Once this is achieved the cat’s response to therapy can be assessed before making a long-term treatment decision.  The risks of any anaesthesia and surgery are also greatly reduced.

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Felimazole® for the treatment of hyperthyroidism

Dechra Veterinary Products has developed Felimazole – the first licensed veterinary drug for feline hyperthyroidism in the UK..

Felimazole comes in the form of small sugar coated tablets containing the active ingredient thiamazole (methimazole).  

Felimazole blocks the synthesis of T4 and T3 within the thyroid gland. It also reversibly blocks activity of the enzyme thyroid peroxidase.

The data sheet’s normal recommended dose is a 2.5 mg tablet given twice a day for 3 weeks initially– the dose should not depend on the weight of the cat. A cat should be re-assessed at 3, 6, 10 and 20 weeks and thereafter every 3 months.  Assessing the response to treatment in this way enables the veterinary surgeon to monitor for side-effects and unrelated complications and conditions.

Felimazole can be used for the long-term treatment of the condition.  Felimazole is also highly effective for pre-operative stabilisation if surgery is the chosen option. In this case, a 3 week course of treatment is recommended.  This improves anaesthetic safety and allows for the unmasking of underlying conditions such as renal failure.

With clinical trials showing 90% success, Felimazole is an effective treatment for feline hyperthyroidism.  Treatment is reversible and side-effects are minimal.

Click here for detailed information about Felimazole.

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