Radioiodine Therapy for Feline Hyperthyroidism

Hyperthyroidism (overactive thyroid glands) is a very common disorder of middle aged to older cats. It is caused by an increase in production of thyroid hormones from the thyroid glands, which are situated in the neck. Clinical signs associated with hyperthyroidism can be quite dramatic and cats can become seriously ill with this condition. However, in most cases hyperthyroidism is treatable and most cats will make a complete recovery.

Typical clinical signs

Hyperthyroidism is almost exclusively seen in middle to old-aged cats, and is rarely seen in cats less than seven years of age. Male and female cats are affected with an equal frequency. The 'classic' signs of hyperthyroidism are weight loss, usually despite an increased appetite (polyphagia), increased thirst (polydipsia), increased irritability, and restlessness or even hyperactivity. Many affected cats have a rapid heart rate (tachycardia) and develop an unkempt hair coat. Mild to moderate diarrhea and/or vomiting is also quite common. Some affected cats will seek out cooler places to sit, and some (especially advanced cases) may pant when they are stressed. Most hyperthyroid cats will show some degree of polyphagia (excessive appetite) and restlessness. In advanced cases there can be generalized weakness, lethargy and loss of appetite.

Reaching a diagnosis

If hyperthyroidism is suspected, a thorough physical examination and some blood tests will help to confirm the diagnosis. On examination, one or two enlarged thyroid glands can often be palpated (felt) as a small, firm mass in the neck (these are often about the size of a pea in hyperthyroid cats). However, in some cats there is no palpable thyroid enlargement, and this can be because the overactive tissue is present in an unusual (ectopic) site (often within the chest cavity).

The diagnosis is confirmed by determination of thyroid hormones in the blood. At Petworks we regularly screen our more mature cats with a general blood chemistry panel and thyroid level as well as urinalysis. In hyperthyroid patients some of these tests may be abnormal – for example liver enzymes are commonly increased secondary to hyperthyroidism, and assessment of routine blood and urine tests can help rule out any other concurrent disease (such as renal failure). We also check blood pressure because hyperthyroid cats often have hypertension. If secondary heart disease is suspected, then an electrocardiogram (ECG – electrical tracing of heart activity), and a chest X-ray or ultrasound may also be performed. In occasional cases, hyperthyroidism may be strongly suspected on the basis of the clinical signs, but blood testing may reveal a normal thyroid hormone (T4) concentration. There are a number of potential reasons for this and usually on a repeat test it will be elevated. If not, additional tests may need to be undertaken to confirm or rule out hyperthyroidism.

Radioactive Iodine Therapy

There are a number of treatments for this disease in cats from oral medications to surgery; however, for most newly diagnosed hyperthyroid cats radioactive iodine (I-131) is the best treatment available. Like surgical removal of thyroid tissue, it has the advantage of being curative in most cases with no ongoing treatment required as with oral medications.

Radioactive iodine is administered as an injection given under the skin – the iodine is then taken up by the active (abnormal) thyroid tissue, but not by any other body tissues, resulting in a selective local accumulation of radioactive material in the abnormal tissues. The radiation destroys the affected abnormal thyroid tissue, but does not damage the surrounding tissues or the parathyroid glands. The advantages of radioactive iodine are that it is curative, has no serious side-effects, does not require an anaesthetic and is effective in treating all affected thyroid tissue at one time, regardless of the location of the tissue. However, it does involve the handling and injection of a radioactive substance. This carries no significant risk for the patient, but precautionary protective measures are required for people who come into close contact with the pet. For this reason, the treatment can only be carried out in certain specially licensed facilities and a treated cat has to remain hospitalized until the radiation level has fallen to within acceptable limits. This usually means that the cat must be hospitalized for between six to ten days following treatment. Most treated cats have normal thyroid hormone concentrations restored within three weeks of the treatment, although in some it can take longer.

A single injection of radioactive iodine is curative in around 95 per cent of all hyperthyroid cases, and in the few cats where hyperthyroidism persists, the treatment can be repeated. Occasionally a permanent reduction in thyroid hormone levels (hypothyroidism) occurs following radioactive iodine treatment, and if this is accompanied by clinical signs (lethargy, obesity, poor hair coat) then thyroid hormone supplementation may be required (in the form of tablets).

Radioactive Iodine treatment is available in Atlantic Canada here at Petworks Veterinary Hospital. We are the only licensed facility east of Montreal to be able to provide this treatment. If you feel your best friend might benefit from I-131 treatment or for more information contact Dr. Rick Swinemar at Petworks Veterinary Hospital 902-435-2444.