Feline Hyperthyroidism

What is hyperthyroidism?

Hyperthyroidism is the most common endocrine (hormonal) disorder of cats. It is most common in older cats, rarely seen in cats under eight years of age. There is no sex or breed predisposition.  It is due to an increase in production and secretion of thyroid hormone by the thyroid gland in the neck usually by benign tumours called “adenomas”. The thyroid gland sets the metabolic rate for the body and as a result, an over-active gland results in wide ranging effects on the cat’s body and vital organs. Hyperthyroidism is very treatable and with some forms of treatment results in a complete cure.

What are the clinical signs of hyperthyroidism?

Cats may present with a combination of the following clinical signs which tend to develop gradually:

1.  Weight loss

2.  Increased appetite

3.  Hyperactivity and restlessness

4.  Moderate elevation of body temperature

5.  Increased heart rate, with a variety of cardiac rhythm irregularities and murmurs

6.  Increased frequency of defaecation, with abundant, bulky stools

7.  Increased thirst and urination

8.  Occasional vomiting

9.  Panting

10.  Matted, greasy and unkempt coat

How is hyperthyroidism diagnosed?

In hyperthyroidism a nodule is usually palpable in one or both of the thyroid lobes in the neck. As the enlarged lobe may be freely movable and can slide along and behind the trachea, it may be difficult to detect, and require careful palpation. In the normal cat, the thyroid lobes are not usually palpable.

Once hyperthyroidism is suspected on the basis of clinical signs, the diagnosis is confirmed by detecting elevated thyroid hormone levels on blood tests. Other laboratory tests may also be abnormal, such as elevation of the liver enzymes. Scintigraphy, a specific type of radioactive scan is available in a few specialist centers. It is an extremely useful method of confirming hyperthyroidism in “occult” cases where blood tests are not conclusive

How can hyperthyroidism be treated?

There are three therapeutic options for the treatment of hyperthyroidism. 

Which treatment option is most suitable for your cat depends on a number of factors and your vet will discuss this with you.

1. Anti-thyroid drug therapy

Anti-thyroid drugs are readily available and economical. They do not destroy thyroid gland, but act by interfering with production and secretion of thyroid hormone. Their use does not result in a cure, but rather controls the condition. The anti-thyroid drug most commonly used is carbimazole. It is usually given twice daily by mouth but in some cats, once daily administration is effective. The thyroid hormone levels are monitored periodically, particularly in the early stages of treatment, to check on the effectiveness. Indefinite treatment will be required to prevent signs from returning.

Mild (and often transient) side effects are seen quite commonly in cats on this medication (~15% of patients), and can include anorexia (inappetance), vomiting and lethargy.  More serious side effects are seen less frequently (~5% of patients) and can include a fall in the number of white blood cells, skin problems, clotting problems, or liver disorder. Blood should therefore be tested routinely to monitor for potential side effects, and in some patients the occurrence of severe adverse reactions may necessitate withdrawal of the drug.

A related drug, “methimazole” is available as a transdermal medication that can be applied to the inner ear and is absorbed across the skin. This method is very useful for cats that are difficult to medicate orally. The absorption of the medication between cats is quite variable and in some cases it does not always effectively control the hyperthyroidism. The medication needs to be formulated by a compound pharmacist and your veterinarian will need to write a prescription for this. This form of medication is more expensive than oral medication. You must always wear disposable rubber gloves when handling this medication as it will be absorbed through your skin if left unprotected. It is also important to avoid direct contact with your cat’s ears. 

"A related drug, “methimazole” is available as a transdermal medication that can be applied to the inner ear and is absorbed across the skin. This method is very useful for cats that are difficult to medicate orally."

2. 131I (radioactive iodine) therapy

Iodine (I¹³¹) therapy involves administering this radioactive element subcutaneously (injected under the skin), or orally to your cat and is only available at licensed facilities. 

I¹³¹  is selectively concentrated within the follicles of the thyroid gland and destroys overactive thyroid tissue, including any areas of thyroid tissue which may be inaccessible to surgery, and spares adjacent normal tissue, including the parathyroid glands.

The primary advantages of ¹³¹I treatment are that it is curative, has no serious side-effects (no toxicity, no hypoparathyroidism), does not require an anaesthetic, is associated with a low recurrence of hyperthyroidism and the location of the tumour is unimportant. In Australia, most capital cities have a facility(s) for treating cats and the cost of treatment is comparable to surgical removal. Because of the reasonable number of facilities in Australia, radioactive iodine is considered the treatment of choice for most cats. Additionally, large doses of ¹³¹I are the only effective treatment for thyroid adenocarcinoma, which is responsible for around 1-2% of feline hyperthyroid cases.

Hospitalisation is mandatory for between 7-10 days following treatment to allow the levels of radiation emitted by the cat and their excretions to fall to a safe level for close contact with owners in their home environment. Radioactive iodine is not suitable for use in patients requiring intensive care as, particularly in the early days following treatment, excessive handling of the cat must be avoided.

3. Surgical thyroidectomy

Surgical thyroidectomy (removal of the thyroid glands) also provides a cure however it requires an anaesthetic which can be a major risk to a hyperthyroid cat.

Anaesthesia can be problematic in hyperthyroid patients both as a direct result of the condition being treated, and also because many patients have concurrent diseases such as chronic renal failure or heart disease. To reduce hyperthyroid-related surgical risks, patients should be pre-treated with anti-thyroid drugs for 3 to 4 weeks prior to surgery to reduce their thyroid hormone levels back to normal. Any associated cardiac disease should be carefully controlled.

Surgery requires special skills in order to avoid serious complications such as damage to the parathyroid gland causing life-threatening hypocalcaemia (low blood calcium levels). In Australia this operation is not commonly performed and your veterinarian may offer you referral to a specialist surgeon – particularly one that works in a 24 hour hospital so that your cat can be appropriately monitored in the critical days following surgery. Recurrence of hyperthyroidism is uncommon if both thyroid glands are removed. A small amount of thyroid tissue is often left behind and cats rarely need thyroid hormone replacement following this form of treatment.