Hyperthyroidism is a relatively common disease seen in cats over 8 years old. In most  cases, it is brought on by a non-malignant growth of the thyroid gland, which causes an increased production of thyroid hormone. Hyperthyroidism is a progressive disease which, if untreated, will continue to worsen until the cat dies.

Clinical signs:

Cats with hyperthyroidism can exhibit some or all of a variety of symptoms which include:

  • weight loss
  • excessive appetite
  • poor hair coat
  • hyperactivity
  • vomiting
  • diarrhoea

How is hyperthyroidism diagnosed?

We will run a blood test in the hospital’s pathology lab to determine the cat’s thyroid hormone level. Additional blood tests should be performed concurrently to screen for other problems that might be present in older cats and which might influence treatment of choice and prognosis.

Treatment protocols:

There are a number of treatment options, each of which has advantages and disadvantages. We will discuss with you the treatment that provides the best possible outcome for your cat.

Medication: ‘Anti-thyroid’ drugs, which block the production of thyroid hormone, can be used to control the signs of hyperthyroidism. These drugs do not cure the disease, but they can effectively control it when given daily. This therapy requires giving the cat tablets every day (initially 2-3 times each day). Medication is the least complex treatment option but it depends entirely on you diligently giving your cat oral medication. If drug treatment is interrupted even for a day or two thyroid hormone levels will increase and symptoms will recur.

Surgery: Surgery can be performed to remove overactive thyroid glands, ‘curing’ the hyperthyroid state. As cats suffering hyperthyroidism are invariably older, there is an increased surgical risk. However, in the majority of cats without any concurrent medical conditions, the risk is minimal. Before surgery, we must first control your cat’s hyperthyroidism with oral medication. This takes 3-6 weeks in most cats.

In the majority of hyperthyroid cats, both thyroid glands must be surgically removed. Although this cures the hyperthyroid state, most cats will also require thyroid hormone supplementation — usually once a day — following surgery, to prevent hypothyroidism, a state of thyroid hormone deficiency (note the different spelling. Hypothyroidism is not the same as hyperthyroidism). These supplements are usually only required for a few months after surgery, but in some cases have to be given for the remainder of the cat’s life.

A serious potential post-surgical complication is hypocalcaemia, or low calcium levels in the blood. This can develop in about 5 to 10% of cats when the small parathyroid glands — located in and around the thyroid glands — are inevitably removed, or when their blood supply is temporarily damaged. Parathyroid hormone is important in regulating calcium levels in the blood. Cats must be kept in the hospital for 2-3 days following surgery so we can watch for signs of hypocalcaemia, which include muscle tremors, stiffness or convulsions. Successful treatment of hypocalcaemia includes daily administration of vitamin D and calcium. In the majority of cats, the medication can be discontinued within 2-3 months.

Radioactive Iodine: This is the least invasive treatment for your cat. Radioactive Iodine (I131) is given as a single tablet or injection. It is effective in correcting the hyperthyroid state in over 85% of cases with a single dose. I131 selectively irradiates abnormal thyroid tissue while preserving the normal thyroid gland. Because of radiation safety requirements your cat will need to be referred to a specialist clinic  and boarded for 7 days after treatment.

Hypothyroidism may also develop following radioactive treatment. However, even though the blood levels of thyroid hormone may be low, the cat often shows no clinical signs of disease and may not need to receive supplementation. Hypocalcaemia is not a complication with this form of treatment.