Hyperthyroidism (thyrotoxicosis) is a common hormone disease of older cats that we have diagnosedwith increased frequency in recent years. It is almost always caused by a benign tumor of the thyroid gland that increases the amount of thyroxine (the hormone secreted by the thyroid gland) in the bloodstream. This increase in thyroxine causes an increase in the overall metabolism of the body, leading to problems for several internal organs. Even though this disease can be diagnosed in young cats, most cats that get this problem are older. Cats 8 years of age or older should be screened for this problem when routine blood panels are run.
Cats that have Feline Hyperthyroidism commonly have other problems that need careful attention if the thyroid problem is to be treated successfully. The excess thyroxine can cause these other problems, or make them worse if they already exist. Some of these other common problems are kidney disease,heart disease, dental disease, sugar diabetes (diabetes mellitus) and high blood pressure (hypertension) leading to blindness.
Just to show how unique each species is, dogs usually get hypothyroidism, the opposite problem with the thyroid gland.
The thyroid is a small and paired gland located at the neck. If enlarged it can sometimes be palpated.
The thyroid gland utilized Iodine in food to produce thyroxine (also known as T-4 or levo-thyroxine), a hormone involved with the bodies metabolic rate. T-4 secreted by the thyroid gland gets converted to T-3 in the liver. It is now called triiodothyronine (T-3), which is the active form. When T-3 circulates through the bloodstream if affects the metabolism of every cell in the body.
The benign nodules that appear on the thyroid gland in this disease secrete excess of amounts of T3 and T4. In most cases both glands are enlarged. These hormones are not under the control of TSH (thyrotropin) secretion.
It is caused by a benign tumor (called an adenoma) of the thyroid gland in almost all cases. This tumor produces excess amounts of thyroid hormone, which circulates through be bloodstream and affects the metabolism of many internal organs.
In rare cases a malignant tumor called a carcinoma is the cause.
The symptoms that occur depend on which internal system or systems are most influenced by the increase in thyroxine circulating throughout the bloodstream. The more common ones are:
It is easy to overlook some of these symptoms, especially if they are subtle. Some people even think of these symptoms as a normal part of the aging process of cats. If left untreated hyperthyroidism can cause heart failure.
There are four primary methods of treating this disease. Each has its advantages and disadvantages, and depends on your unique situation. One of our doctors will discuss which option is best used in your case.
Prescription Diet Y/D
This new food from Hills limits the amount of iodine to between 0.17 ppm to 0.3 ppm (ppm is parts per million, which is obviously very little). All other cat foods have 1.5 ppm to 99 ppm of Iodine in them. It has been found that this reduction in iodine prevents the diseased thyroid gland from producing excess thyroxine. T4 levels stay normal and thus there are no symptoms. This food is made for older cats also that might have other diseases like kidney disease because it has limited phosphorous for the kidneys with extra omega -3 and omega-6 fatty acids. If your cat eats this food well this is all you need to treat the disease.
We recommend all cats that are currently on Tapazole (Methimazole) give this a try. Before changing your cat over we follow a specific protocol:
Gradually introduce the food over 7 days by mixing it in with the regular food.
Reduce the dose of Tapazole by 50% over this 7 days.
If your cat is eating the Y/D well exclusively at the end of the second week then stop Tapazole completely.
Bring your cat in for an exam and a blood panel with T4 level 4 weeks after starting Y/D. An exam is needed to check weight, listen to the heart with a stethoscope for murmurs, determine heart rate and blood pressure to make sure these problems that are common with Feline Hyperthyroidism are not present on just the Y/D food. In addition to checking the T4 level the blood panel checks for other problems common in older cats, especially the kidneys.
You cannot feed any other food, especially tuna, while your cat is on Y/D. Almost all foods contain excess amounts of Iodine, including some flavored chews, pills, and vitamins.
If left untreated there are significant complications that can develop. Blindness can occur due to retinal detachment from high blood pressure. Long term kidney damage and non-stop diarrhea might also be consequences, along with heart failure and death.