Hyperthyroidism (overactive thyroid glands) is a 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. Clinical signs associated with hyperthyroidism can be quite dramatic. However, in most cases hyperthyroidism is treatable and most cats will make a complete recovery.
Thyroid hormones have an important role in controlling the body’s metabolic rate and also the general activity level, so cats with hyperthyroidism tend to burn up energy too quick and typically suffer weight loss despite having an increased appetite and increased food intake.
In the vast majority of cases the increased thyroid hormone production is due to a benign (non-cancerous) change. Both of the thyroid glands are involved, although one gland may be more severely affected than the other. The abnormal thyroid tissue becomes enlarged, but the underlying cause of this change is currently unknown. Cats usually respond extremely well to treatment, and if the condition is recognised early and treated appropriately, then the outlook for the affected cat is generally very good.
A malignant (cancerous) tumour known as a thyroid adenocarcinoma can also be an underlying cause of some cases of hyperthyroidism. Fortunately this is rare.
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.
Cats affected with hyperthyroidism usually develop a variety of clinical signs, which are often quite subtle at first, but then become more severe as the disease progresses.
The ‘typical’ signs of hyperthyroidism are weight loss, usually despite an increased appetite, increased thirst, increased irritability, and restlessness and hyperactivity. Many affected cats have a rapid heart rate and develop an unkempt coat. Diarrhoea and/or vomiting is also quite common. Some cats will be noticeably intolerant of heat and seek out cooler places to sit, and some may pant when they are stressed.
Thyroid hormones have effects on virtually all the organs in the body, and therefore this disease can sometimes cause secondary problems that may lead to additional investigations and treatment.
The effect of thyroid hormones on the heart is to stimulate a faster heart rate and a stronger contraction of the heart muscle. Over time the muscle of the largest chamber in the heart (the left ventricle) enlarges and thickens. If left untreated and unmanaged, these changes will eventually compromise the normal function of the heart and can result in heart failure. This means that in some cats with hyperthyroidism, additional treatment may be required to control secondary heart disease.
High blood pressure is another potential complication of hyperthyroidism and can cause additional damage to several organs including the eyes, kidneys, heart and brain. If high blood pressure is diagnosed along with hyperthyroidism, drugs will be needed to control the blood pressure to reduce the risk of damaging other organs.
Kidney disease (chronic renal failure) does not occur as a direct effect of hyperthyroidism, but the two diseases often occur together because they are both common in older cats.
Reaching a diagnosis
If you or your veterinary surgeon suspect hyperthyroidism, a thorough physical examination and blood tests will be required to confirm the diagnosis.
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.
There are three main options for the treatment of hyperthyroidism, each with advantages and disadvantages:
Medical management (drug therapy)
Anti-thyroid drugs are available in tablet form and these act by reducing the production and release of thyroid hormone from the thyroid gland. They do not provide a cure for the condition, but they do allow either short-term or long-term control of hyperthyroidism.
Anti-thyroid drug treatment has the advantage of being readily available and economical, but it is not curative. Life long treatment will be required and for some owners, and some cats, this may be difficult to achieve. Routine blood tests should be checked periodically during treatment to monitor the effectiveness of therapy.
Surgical removal of the affected thyroid tissue (thyroidectomy) can produce a permanent cure and is a common treatment for many hyperthyroid cats. In general this is a very successful procedure and is likely to produce a long-term cure or permanent cure in most cats.
To reduce anaesthetic and surgical complications, where possible it is always recommended that hyperthyroid patients are initially stabilised with anti-thyroid drugs for three to four weeks before surgery. Any associated heart disease must also be treated where necessary. The major risk associated with surgery itself is inadvertent damage to the parathyroid glands – these are small glands that lie close to, or within, the thyroid glands themselves, and have a crucial role in maintaining stable blood calcium levels. Damage to these glands can result in a life-threatening fall in blood calcium concentrations (hypocalcaemia).
Radioactive iodine therapy
Radioactive iodine (I 131) is a very safe and effective cure for hyperthyroidism wherever the location of the overactive thyroid tissue. It has the advantage of being curative in most cases with no ongoing treatment required.
Radioactive iodine is administered as a single 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 cat. For this reason, the treatment can only be carried out in certain specially licensed facilities and a treated cat has to remain hospitalised until the radiation level has fallen to within acceptable limits. This usually means that the cat must be hospitalised for between three and six weeks (depending on the facility) following treatment. Most treated cats have normal thyroid hormone concentrations restored within three weeks of the treatment, although in some it can take longer.
Hyperthyroidism is one of the most common cat illnesses and it is curable in many cases. ‘Caring for a cat with hyperthyroidism’ has been written as an information source and support tool primarily aimed at owners whose cats have been diagnosed with this condition. It is designed to be a resource for student vets, veterinary nurses and technicians. The book explains what hyperthyroidism is, how it is diagnosed and what the various treatment options are. The medical and emotional aspects of coping with this condition are discussed with practical advice on all aspects of care. The book contains a detailed glossary of terms used by vets and several case reports illustrating how successful treatment can be