Feline Hyperthyroidism
WHAT IS HYPERTHYROIDISM?
Hyperthyroidism is the most common endocrine (hormonal) disorder of cats. It is rarely seen in cats under eight years of age, and 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.
WHAT ARE THE CLINICAL SIGNS OF HYPERTHYROIDISM?
Cats may present with a combination of the following clinical signs, which tend to develop gradually:
- Weight loss
- Increased appetite
- Hyperactivity and restlessness
- Increased heart rate, with a variety of cardiac rhythm irregularities and murmurs
- Increased frequency of defecation, with abundant, bulky stools
- Increased thirst and urination
- Occasional vomiting
- Panting
- Matted, greasy and unkempt coat
HOW IS HYPERTHYROIDISM DIAGNOSED?
In hyperthyroidism a nodule is often palpable in one or both of the thyroid lobes. 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 either not palpable or small and symmetrical.
Once hyperthyroidism is suspected on the basis of clinical signs, the diagnosis is confirmed by performing a blood test that specifically detects elevated serum thyroid hormone levels. We also recommend running a general health profile (blood test) to check other aspects of your cat’s health such as kidney and liver function, and presence of anaemia.
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.
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 most commonly used drug is called carbimazole and will need to be given to your cat for life.
Most cats are initially started on twice daily dosing, and then after three weeks their thyroid hormone levels and kidney function are checked to see the response to therapy. This will then allow more fine-tuning of your cats dose rate to suit his/her condition. Once the thyroid hormones are at a stable level, we recommend repeat blood tests every 3-6 months, or sooner if you notice you cat losing weight/eating more.
For a useful video on how to give your cat a pill, please refer to our information page: "Giving Medicine to Your Cat."
Mild (and often transient) side effects are sometimes seen in cats on this medication (~15% of patients), and can include poor appetite, 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, clotting problems, or liver disorder. Blood should therefore be tested routinely to monitor for potential side effects, and occasionally in some patients the occurrence of severe adverse reactions may necessitate withdrawal of the drug.
For cats that will not take medication orally, anti-thyroid drugs are now available formulated in a transdermal gel that is applied into the inner aspect of the ear. The drug absorbed through the skin into the blood stream. This formulation costs more than the tablets but can be as effective at stabilising the disease.
If your cat also has a very fast heart rate (a common symptom of hyperthyroidism) you vet may prescribe a course of cardiac medications to reduce the heart rate until the medication becomes effective.
Surgical Thyroidectomy
Surgical thyroidectomy (removal of the thyroid glands) has the immediate advantage over drug therapy in that it provides a cure. This treatment is available, although surgery presents a moderate risk to the patient.
Anaesthesia can be problematic in hyperthyroid patients both as a direct result of the condition being treated, and also because a number of patients have other concurrent diseases e.g. chronic renal failure. 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.
Side effects of the surgical procedure may include nerve damage, or hypoparathyroidism (lack of the hormone that controls the level of calcium in the blood). The parathyroid glands are located very close to the thyroid glands, and so can be easily damaged when the thyroid glands are being removed. The resultant hypocalcaemia (low blood calcium level) can result in muscle twitching, weakness and convulsive seizures. Patients in the veterinary practice are observed closely for the first 2-3 days after surgery.
There is generally a low rate of recurrence of hyperthyroidism following surgery, although some cases do recur. This can happen when a case of bilateral hyperthyroidism (i.e. where both thyroid lobes are affected) is mistakenly treated as a unilateral case (where only one side is affected) - the differentiation of normal from abnormal thyroid tissue is not always straightforward. Around 70% of hyperthyroid cases are bilateral, and in unilateral disease the gland on the opposite side of the neck is normally reduced in size. Occasionally, adenocarcinoma (malignant tumours) are present, and although they do not usually spread through the body, local invasion may prevent satisfactory surgical excision.
I131 (radioactive iodine) therapy
This uses radioactive iodine (I131) which is administered by a specialist veterinarian. I131 selectively destroys the affected 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 I131 treatment are that it is curative, has no serious side effects, does not require an anaesthetic or sedation, and is associated with a low recurrence of hyperthyroidism.
However, the cost of treatment is much greater than either medications or surgery, and currently this procedure can only be performed at a specialist facility. Patients with underlying kidney disease are not ideal candidates for this treatment as they may experience exacerbation of their symptoms. Patients also require boarding for several weeks while they remain radioactive from their treatment. Currently there is no veterinary facility offering I131 treatment in Western Australia.