Hyperthyroidism in Cats
Causes and Treatment of Endocrine Diseases in Felines
Hyperthyroidism is a condition in which the endocrine system overproduces the hormones thyroxine (T3) and triiodothyronine (T4). This is usually caused by a tumour within the endocrine system, though occasionally the tumour may be elsewhere in the body.
Cats are most likely to succumb to hyperthyroidism when they are more than 8 years old. If untreated, hyperthyroidism is fatal.
Canned Foods and Hyperthyroidism
Canned food has been implicated in the increased prevalence of hyperthyroidism due to the Bisphenol-A-diglyciddyl ether used to line pop-top catfood cans. Cats that eat only canned food from pop-top style tins are 5 times more likely to develop hyperthyroidism, and cats that receive 50% of their food from these tins are 3.5 times more likely to develop the disease. However, one-quarter of cats that have hyperthyroidism have never eaten canned food, so even if the food tins play a role, they are not the only cause. Also, purebreds are less likely to be afflicted with hyperthyroidism, which suggests that the condition has a genetic component.
Symptoms
Symptoms may range from mild to severe, depending on how excessive the hormone production has become. Common symptoms include:
Rapid, noticeable weight loss despite eating voraciously
- Hyperactivity
- Nervousness
- Dull, ungroomed coat
- Rapid heartbeat
Less common symptoms include:
- Increased consumption of water
- Vomiting due to overeating
- Panting or other breathing difficulties
- Diarrhea
Treatment
Treatments for hyperthyroidism are surgery, medication and radioactive iodine therapy. The pros and cons of each are outlined below:
Surgery – In some cases, only one thyroid gland needs to be removed. However, if both require removal, the cat will need hormone replacement therapy daily for the rest of her life and undergo regular thyroid level monitoring (twice a year). A common complication of surgery is hypoparathyroidism. This condition, which occurs when the surgeon accidentally damages or removes the parathyroid glands, creates low blood calcium levels. Symptoms include extreme weakness, muscle spasms or tremors, and seizures in some cases. Cats with hypoparathyroidism require calcium and vitamin D supplements for the rest of their lives.
Medication – This is the least expensive option but in addition to requiring daily administration, medication can cause serious side effects including anemia, vomiting, itching, weakness, decreased appetite and immune cell suppression. If medication is chosen, cats must stay on it for the rest of their lives.
Radioactive Iodine Therapy – This treatment, which uses radiation to selectively destroy malfunctioning thyroid cells, costs approximately $1,000-1,700 in North American and 500-900 pounds in the UK. Cats generally have to stay at a medical facility for 7 to 25 days, until the radiation in their bodies is at a safe level for them to interact with people and other animals, and owners cannot visit their cats during this time. Radioactive iodine therapy is very effective, completely curing cats in approximately 98% of cases. Some cats that have undergone radioactive iodine therapy require hormone supplements for the rest of their lives, while others need no further treatment.
Hypothyroidism
Hypothyroidism occurs when the endocrine system does not produce enough hormone, or any at all. The endocrine system may be attacked by the cat’s immune system, which can mistake the cat’s own tissue for foreign tissue and destroy it. In many cases, symptoms do not appear until the system has been completely destroyed. Hypothyroidism can also be caused by a failure in the feedback system; another gland may fail to produce sufficient quantities of a hormone that signals the endocrine system to produce another hormone.
Hypothyroidism is treated by replacing the missing hormone. This can be done using oral medication in most cases. Cats receiving hormone supplements should be monitored closely with regular veterinary check-ups to ensure that the dose is right and that the cat is not suffering adverse side effects.
Other Endocrine System Diseases
There are a number of other endocrine system diseases that can afflict cats:
- Hyperadrenocorticism (Cushing’s Disease) – Extremely rare, this disease causes increased appetite, urination and water consumption, as well as hair loss and a pot-bellied appearance.
- Hypoadrenocorticism (Addison’s Disease) – Also rare, this disease is usually caused by immune-suppressant drugs.
- Hypoparathyroidism – In addition to occurring as a result of accidental parathyroid gland removal, this disease can be caused by diets high in phosphorus (organ meats such as liver, kidney, etc.) or kidney disease.
- Diabetes Mellitus – Increasingly common in cats due to obesity, symptoms of this disorder of glucose metabolism include increased water consumption and urination. In the long run, metabolic difficulties can lead to liver damage, depression, rear-limb weakness and even coma, as well as increased risk of chronic bladder and skin infections. Treatment includes insulin injections and strict feeding schedules, and, in complicated cases, hospitalization with intravenous support.
Further Reading
For more on cat care, see Cat Nutrition and Home-Made Food and Cats and Furniture Scratching.
References:
- Brooks, Wendy C., DVM, DABVP. (2008). “Signs, Symptoms and Diagnosis of Hyperthyroidism.”
- Merck & Co., Inc., Eds. Cynthia M. Kahn, BA, MA & Scott Line, DVM, PhD, Dipl ACVB. (2007). The Merck/Merial Manual for Pet Health, Home Edition.
- Eldredge, D.M.,DVM, Carlson, D.G., DVM, Carlson,L.D., DVM & Giffin, J.M., MD. (2008). Cat Owner’s Home Veterinary Handbook, Third Edition. Wiley Publishing, Inc.
