Hyperthyroidism in cats
The most common hormonal disorder seen in cats today is an overactive thyroid gland known as hyperthyroidism. While it was only recently characterized and identified as an illness in cats in 1979 at the Animal Medical Center in New York City, the incidence of this disorder has expanded tremendously over the past three decades. While it is most commonly a disease of cats over the age of ten, we are now identifying this disorder in younger and younger cats due to more accurate testing, including measurement of both a T4 and Free T4 thyroid levels.
The most common cause of this hormonal imbalance is due to a benign but functional tumor of the thyroid glands that leads to an overproduction of thyroid hormone. The debate in the veterinary community rests on what causes this benign tumor to develop. The largest consensus rests on environmental factors, including potentially contaminant metals in certain screw top canned cat foods, to environmental toxins in carpets, as well as over vaccination where immunity to most viruses lasts for years to life of the pet.
As with most chronic disorders in our pets, it is believed to be an autoimmune reaction, where the immune system reacts against its own tissues leading to the development of this functioning tumor of the thyroid glands. Symptoms of this disease include increased thirst/urination and appetite with concomitant weight loss at the same time. Greasy or matted coats including excessive shedding can develop as well. Behavioral changes include easy overheating and panting, restlessness, vocalization, aggression and even changes in litter box habits resulting in feline house soiling. If left undiagnosed, high blood pressure, subsequent kidney and heart damage, and secondary disease of the heart muscle called cardiomyopathy can develop.
Treatment options include the prescription drugs Tapazole or generic Methimazole. 15 to 20% of cats may develop side effects on these drugs including lethargy, loss of appetite and vomiting. A lesser percentage can develop problems of the bone marrow, liver failure and even less common intense itching of the head and face. In roughly 20% of cats treated with Methimazole, elevation of kidney enzymes and possibly future renal failure can be seen. That’s why it’s so important for cats treated with medications for this condition to be adequately monitored through periodic exams, blood testing and blood pressure measurements. And while surgical removal of the thyroid glands used to be a popular treatment option, it has for the most part fallen out of vogue because of the close proximity of the parathyroid glands, as well as the surgical risk in many older cats.
The preferred method of treatment by many feline experts includes the injection of a single bolus of radioactive iodine, which selectively destroys only the functioning tumor of the thyroid glands. Most cats tolerate this treatment quite well, with the only downsides in my opinion being the initial expense of $1200 to $1500 dollars for the workup before the treatment, as well as the cat being isolated for at least a few days away from the guardian during the treatment period, in order to avoid radioactive exposure of the human guardian family.
As with all other hormonal diseases, I always recommend good vitamin supplementation, including Soft Vitachews for Cats, Vetri-DMG, fatty acids such as Be Well Cat or Nordic Naturals Pet Cod Liver Oil, as well as liver support such as Denamarin or Denosyl, in order to protect the liver against the toxic effects of long term drug therapies.