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Hyperthyroidism (overactive thyroid glands) is a very common disorder of older cats. It is caused by an increase in production of thyroid hormones (thyroxine) from the thyroid glands,
which are located in the neck (one either side of the windpipe). Despite sometimes causing serious clinical signs, most cases of hyperthyroidism are treatable and most cats will make a complete recovery.
Thyroid hormones have an important role in controlling the body's metabolic rate and thus the general activity level, so hyperthyroid cats tend to burn up energy too rapidly and therefore suffer weight loss despite having a ravenous appetite and increased food intake.
In most cases the increased thyroid hormone production is caused by a benign change to both of the thyroid glands, although one is usually more severely affected. The thyroid gland then becomes enlarged.
Some cats may develop secondary heart disease because the heart has been stimulated to beat faster and harder for a long period of time. High blood pressure (hypertension) is another complication which can in-turn damage other organs including the eyes, kidneys, heart and brain. Treatment may be required to lower the blood pressure.
Kidney disease (chronic renal failure) does not occur as a direct effect of hyperthyroidism, but both are common in older cats and often occur together. Where this is the case, care is needed since hyperthyroidism tends to increase the blood supply to the kidneys, which may improve their function. Blood tests to assess kidney function may show normal or only mild changes, but potentially more severe kidney failure may be masked by the hyperthyroidism. Cats are therefore started on treatment (tablets initially) and their response monitored with blood and urine tests.
If your vet suspects hyperthyroidism, a thorough physical exam and some blood tests will be required to be performed to confirm the diagnosis. On physical exam, one or two enlarged thyroid glands can often be palpated as a small firm mass in the neck, although sometimes this
is hidden in the chest cavity. Elevated thyroid hormones in the blood is the most common finding.
Technetium scanning is a technique available at specialist referral centres to diagnose the exact
location of the abnormal thyroid tissue.
There are three main options for the treatment of hyperthyroidism, each with advantages and disadvantages.
1. Medical management (drug therapy) – these drugs act by reducing the production and release of throxine from the glands. They do not cure the condition but they allow control.
Thyroid hormone levels usually fall to within normal limits within 3 weeks. Treatment needs to be given for the rest of the cat’s life. Regular blood checks are required to check thyroxine levels.
2. Surgical thyroidectomy – surgical removal of the affected thyroid tissue can produce a permanent cure and is a common treatment. However, surgery will not be successful if ‘ectopic’ (extra) thyroid tissue is present and occasionally signs of hyperthyroidism can return at a later time if previously unaffected tissue becomes diseased. Patients must initially be stabilised with drugs before surgery is carried out to reduce anaesthetic and surgical complications. During surgery there is a risk of damage to the parathyroid glands (small glands which lie close to or within the thyroid glands and have a crucial role in maintaining blood calcium levels).
3. Radioactive iodine therapy – is a safe and effective cure for hyperthyroidism no matter where the overactive tissue is located. It has the advantage of being curative in most cases with no ongoing treatment required. The radioactive iodine is given as a single injection under the skin the iodine is taken up by abnormal thyroid tissue but not by any other body tissues. The radiation destroys the affected abnormal thyroid tissue but does not damage the surrounding tissues or parathyroid glands. This type of treatment can only be carried out at certain specialist centres which have the facilities to deal with such radioactive substances. The patient must also stay in hospitalised until the radiation level has fallen to an acceptable level. This may mean hospitalisation for 3-6 weeks.