Why is my old cat losing weight? Part 3 | Companion Animal Veterinary Hospital
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Why is my old cat losing weight? Part 3

The signs of hyperthyroidism are increased appetite and weight loss. There is an old tabby cat's head looking to the lower right.


Hyperthyroidism is the overproduction of the thyroid hormones T3 and T4 from the thyroid glands. It is usually caused by a non-cancerous tumor of the gland, the cause of these tumors are not known but environmental and dietary factors may play a role.

Thyroid hormones influence many of the bodies organs so the effects of overproduction are felt throughout the body.

The signs

Initially the signs are subtle but progress over time. Affected cats:

  • lose weight
  • have increased appetites and
  • drink and urinate more
  • Vomiting and diarrhoea are often seen
  • coats may be unkempt and
  • agitation and hyperactivity are also a feature for many affected cats.
  • Internally the heart is over stimulated, leading to enlargement and
  • the blood pressure is often elevated which causes damage to many organs.

Diagnosis is by a combination of physical exam findings and blood tests. Most cat will have high levels of T4 in the blood, however a small percentage have a normal T4 and require additional test for confirmation. Blood test are often repeated for monitoring treatment and are also useful to identify the health of other organs in the body (like the kidneys) that may have been damaged by the high blood pressure.

Treatment for hyperthyroidism is either, daily medication, surgery, radioactive Iodine or dietary. The pros and cons of these treatments are best discussed with your vet.

The prognosis for this condition is generally good, however the damage caused by high blood pressure may not be reversible and may compromise recovery in some cats. Early diagnosis and treatment are therefore likely to result in better outcomes

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