Hyperthyroidism in Cats

The number of cases of hyperthyroidism in cats has been steadily increasing over the years, it occurs when there is an excess of thyroxine (T4) and tri-iodothyronine (T3) produced by the thyroid gland. Hypothyroidism is when there is a deficiency of these hormones. The thyroid gland controls metabolism. This is a very common endocrine disorder in middle age-older cats but is rarely seen in dogs. There seems to be no sex disposition, however Siamese and Himalayan breeds appear less likely to develop the disease.

Hyperthyroidism is most commonly caused by small benign tumour in either one or both of the thyroid glands, the changes are benign but the reason behind it is still unknown. Rarely, a thyroid carcinoma may be the cause. If left untreated, hyperthyroidism can result in changes in metabolic rates and may cause cardiac abnormalities.

It is important to know that many other diseases of older cats can mimic the symptoms of hyperthyroidism e.g. Diabetes mellitus and IBD.

IODINE LEVELS: At the North American Veterinary Conference 2005 Dr. P. Schenck showed studies which suggested that cats fed on a canned food were more at risk from hyperthyroidism, this may because of substances in the lining of the can or because of iodine levels. Iodine deficiency in cats can cause the thyroid glands to be heavier and larger, cats can be fed on very high or very low iodine levels and can adapt quite well. However canned foods have a much larger range of iodine levels than dried foods and the theory is that variability in the levels of iodine intake by constantly switching brands of canned foods (including flavours in the same brand) can overwhelm the system and induce hyperthyroidism.

Other goitrogens (agents that cause thyroid enlargement) include: sorghum, Soya beans, ascorbic acid and copper sulphate.

There are no detailed studies to determine what level of iodine is consistent with normal thyroid function over a long period of time.

CAT LITTER: According to David Bruyette speaking at the WSAVA 2001, researchers have found a marked increase in hyperthyroidism in cats which use litter trays. Unfortunately, they cannot explain this as yet. More research is needed to determine whether the type of litter is an important factor.

Symptoms

• Polydipsia (Excessive drinking) In 10% of hyperthyroid cats:
• Polyuria (Excessive urination) - depression
• Polyphagia (Excessive appetite) - muscle weakness
• Weight loss - anorexia
• Hyperactivity This is known as ‘apathetic’ hyperthyroidism
• Irritability
• Aggression
• Vomiting
• Diarrhoea
• Voluminous fatty faeces
• Heat intolerance
• Panting
• Skin lesions, dry, greasy, matted coat, alopecia

Heart abnormalities are often present
Hyperthyroidism often masks concurrent renal failure
Liver enzymes are often raised in hyperthyroid cats
Proteinuria is often seen in hyperthyroid cats (may indicate renal failure)

Treatment

There are three main ways of treating hyperthyroidism:

(1) Antithyroid drugs. This treatment relies on the owner giving daily oral medication. Some side-effects may occur (e.g. vomiting) but these are generally seen only in the first few weeks of treatment. Symptoms will reoccur if the treatment is stopped for more than 24-72 hours as this is not a cure, it just controls the disease.

(2) Thyroidectomy: This involves removal of the thyroid glands. Both glands should be removed, even if one looks normal (otherwise symptoms seem to reoccur 6-12 months later). This is a very effective procedure and is quite simple to perform. However, the operation itself may be risky because of the use of anaesthetic with cardiac, renal, hepatic and gastrointestinal problems that may be associated with the hyperthyroidism. This surgery is most successful after the cat has been stabilised with medication for a few months.

(3) Radioactive Iodine: Radioiodine destroys the overactive thyroid cells without damaging the normal ones. This is a very effective treatment but it is expensive and not always widely available as special materials, facilities and licences are needed.

 

Common pet health problems
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