Hyperthyroidism is a common disorder of older cats. Owners complain that their cats have gone crazy climbing curtains and rooting in rubbish bins. An overactive thyroid gland produces and secretes too much thyroid hormone putting pressure on all body organs. Any sex or breed of cat can be affected.
What are the signs of hyperthyroidism?
Any of the following:
1. Weight loss
2. Increased appetite
3. Hyperactivity and restlessness
4. Increased heart rate, associated with irregularities in rhythm, murmurs and high blood pressure
5. Increased frequency of defecation with occasional accidents inside
6. Increased thirst and urination
7. Occasional vomiting
9. Matted, greasy and unkempt coat
10. Enlarged thyroid glands
If we suspect that your cat has hyperthyroidism we send blood for measurement of the thyroid hormone, T4. We also check for secondary liver, heart or kidney problems.
How do we treat hyperthyroidism?
1. Anti-thyroid drug therapy
Anti-thyroid drugs interfere with the production and secretion of thyroid hormone. They control rather than cure the hyperthyroidism.
Carbimazole (brand name Neo Mercazole) tablets are given twice daily. We see mild and often transient side effects in a few cats including poor appetite, vomiting and lethargy. Rarely we see more serious side effects including a fall in the white blood cell count, clotting problems, or liver disorders. If we find any serious side effects after 2 weeks of medication we switch to another treatment.
Many owners prefer methimazole as a gel applied to the inside of the ear or made up into a palatable liquid.
2. Radioactive iodine therapy
If kidney problems are not uncovered by anti-thyroid therapy then we suggest radioactive iodine, a more permanent cure for hyperthyroidism. Radioactive iodine destroys the affected thyroid tissue and leaves adjacent normal tissue, including the parathyroid glands, untouched.
We refer you to a special facility at Canberra Veterinary Hospital for radioactive iodine therapy. Treated cats are kept in hospital for a week after the treatment until they are no longer a radiation risk to people.
Radioactive iodine treatment has no serious side-effects. Depending on the age of the cat at diagnosis the cost of treatment is similar to long term anti-thyroid drugs or surgery.
Radioactive iodine is the only effective treatment for thyroid adenocarcinoma, the cancer that causes 1 to 2% of feline hyperthyroid cases.
3. Surgical thyroidectomy
Removal of the thyroid glands provides an immediate cure but has some nasty potential side effects so we don’t often recommend it.