HYPERTHYROIDISM IN CATS, continued
The second of three posts on hyperthyroidism in cats. The first post discusses the function of the thyroid and the causes and effects of hyperthyroidism. The third post will discuss radioactive iodine treatment.
Hyperthyroidism may be treated by medication which suppresses thyroid hormone production, by surgically removing part of the thyroid gland, by feeding a diet deficient in iodine or by administration of radioactive iodine.
Tapazole (methimazole) is the only medication recommended for treatment of hyperthyroidism in cats. White blood cell and platelet disorders occur in 15% of treated cats but usually disappear without treatment or the need to discontinue the medication. 10% of cats have vomiting, poor appetite and lethargy. This is also generally temporary. Rare (below 5%) side effects that require stopping the medication include facial scratching, bleeding, liver problems, low platelets and anemia caused by destruction of the red blood cells by the immune system (immune hemolytic anemia). Surfeit is one of the rare kitties that get liver problems with Tapazole. His T4 became normal but his bilirubin went up and so did his liver enzymes. He had to stay in the hospital on IV fluids overnight and we are now giving him SQ (subcutaneous) fluids every other day. The good news is that his kidney function did not get worse when his T4 got better, so we are hoping it will stay good after the I-131.
Iopanoic acid is an X-Ray contrast material that suppresses the thyroid as a side effect. There is very little experience with its use in cats and what there is does not suggest it’s a realistic long-term solution.
Surgical removal of part of the thyroid gland (thyroidectomy) is an effective treatment of hyperthyroidism but there are several caveats. One is, as with all surgery, the risk of anaesthesia. The second is that sometimes there is functioning thyroid tissue located in the elsewhere in the body. This ectopic tissue is usually in the chest but can be anywhere and cannot be found on routine thyroidectomy. Thus sometimes enough thyroid tissue is left after the surgery for the cat to continue to be hyperthyroid. Finally, if the surgeon removes most of the thyroid gland, there is the risk of accidentally removing or causing injury to the parathyroid glands, which would cause major problems. For all those reasons, surgical removal of the thyroid is no longer the treatment of choice in most uncomplicated cases.
Hill’s Prescription Diet y/d Feline Thyroid Health is a new medical food for cats which contains a reduced amount of iodine. Since all thyroid hormones contain iodine (see the diagrams in the previous post), if you don’t give the cat iodine, it can’t produce thyroid hormone and it won’t be hyperthyroid. (The y/d has a precisely controlled amount of iodine so the cat can make just enough but not too much thyroid hormone.) The trouble is that if the hyperthyroid cat gets any other food, the benefit of the medical food is negated. So no treats, no snacking from other cats’ bowls, no hunting for outdoor cats. That’s a difficult management problem for multi-cat households. For us, it wouldn’t be practical because my fursib Java has allergies and we all eat a venison and pea hypoallergenic food because of that. There are also concerns about the nutritional value of y/d as the sole source of nutrition for a cat.
Radioactive iodine treatment is the method of choice for treating hyperthyroidism in cats. It works because the thyroid gland is very efficient at concentrating iodine from the bloodstream (just look at all those iodine atoms in thyroid hormones), so when you give radioactive iodine, it goes right to the thyroid and kills the cells by its radiation. The treatment does not affect other organs because the thyroid fishes all the iodine out of the blood stream and holds onto it.
To be continued…
The next post will discuss radioactive iodine treatment: how it’s done and what to expect afterwards.
SOURCES AND FURTHER READING
Wikipedia links and other links above
Hyperthyroidism in Cats by Cornell Feline Health Center
The Merck Veterinary Manual, Merck & Co., Inc., 2005
Plumb, Donald, Plumb’s Veterinary Drug Handbook, Sixth Edition, Blackwell Publishing, 2008
Feline hyperthyroidism by Advanced Veterinary Medical Imaging
What Is Feline Hyperthyroidism? by Cat Thyroid Center
http://www.endocrinevet.blogspot.com/ — various articles
Erick Mears, DVM, personal communication