Hyperthyroidism is the most common hormone disorder of cats. It is caused by growths within the thyroid gland that are overactive, leading to excess thyroid hormone in the body. Thyroid growths are usually benign but can be malignant in rare cases.
Common symptoms of hyperthyroidism include hyperactivity, weight loss and hunger. Some cats will become more vocal, thirsty and may also urinate more than usual. Occasionally, cats will not show any symptoms of hyperthyroidism but the disorder can be detected on wellness testing by your veterinarian.
Fortunately for your cat, this is a very treatable problem with several management options:
1) Methimazole (generic, Tapazole TM, FelimazoleTM) is a medicine that inhibits the production of thyroid hormone, thereby resolving symptoms of hyperthyroidism. This treatment is highly effective, readily available, and not very costly in the short term. It can be administered orally as a pill or liquid, or topically on the skin. This treatment must be monitored periodically to make sure it is working properly and not causing side effects. It is important to note that this treatment does not cure hyperthyroidism, but can control the problem as long as the medicine is given regularly as prescribed by your veterinarian.
2) Surgery can be performed to remove the abnormal thyroid tissue. This treatment has been effective in curing hyperthyroidism in many cats but is less popular recently as safer and simpler options are now readily available.
3) I-131 (radio-iodine) therapy is a form of nuclear medicine used to cure feline hyperthyroidism. This is a very simple, safe and highly effective treatment. The vast majority of cats treated with I-131 will be cured of hyperthyroidism.
How does I-131 work?
The thyroid gland is the only part of the body that uses iodine, a nutrient that is essential to synthesize thyroid hormones. I-131 is a form of iodine that is radioactive. I-131 is given by injection and circulates in the blood until it is taken into the thyroid gland. In hyperthyroid cats, the cells within the abnormal nodules are actively making an abundance of thyroid hormone, but the normal thyroid tissue is suppressed and not making any thyroid hormone. Therefore only the cells within the nodules will absorb the radioactive iodine. Once inside the abnormal thyroid cells, the radiation goes to work to destroy the abnormal cells. Small amounts of radiation will be emitted from your cat during the treatment, therefore strict isolation in our facility is required to keep everyone safe. Once the level of radioactivity is below a certain limit, usually within a few days, the cats are safe to go home.