Summary
Hyperthyroidism is an endocrine disease caused by increased making and release of hormones by the thyroid gland, which is a small, two-lobed organ that sits in the middle of the neck. Hormones regulate and influence bodily functions. Thyroid hormones help regulate the body's metabolism by affecting heat production and the body's use of nutrients (protein, carbohydrates, and fat). Thyroxine, also called T4, is the main hormone produced by the thyroid gland.
Endocrine systems are hormone-secreting organs. Other examples of endocrine diseases are diabetes mellitus (caused by a malfunctioning pancreas) and hyperadrenocorticism (Cushing disease, caused by malfunctioning adrenal glands).
Hyperthyroidism is the most common endocrine disease of cats. Hyperthyroidism can also occur in people and is called Grave's disease or toxic nodular goiter.
Hyperthyroidism is caused by enlargement of the thyroid gland (called thyroid hyperplasia), noncancerous thyroid tumor in the thyroid gland (thyroid adenoma), or thyroid gland cancer (thyroid adenocarcinoma). In dogs, cancer is the most common cause. Feline hyperthyroidism is more commonly caused by thyroid hyperplasia or thyroid adenoma.
Symptoms and Identification
Extreme weight loss and constant hunger are the most common symptoms, but others include:
Increased Vocalization (e.g. "yowling" in a cat)
Aggressive Behavior
Hyperactivity
Lump on cat's neck (where the thyroid gland is located)
A hyperactive thyroid can also affect the heart (e.g. altered rhythm, murmurs, fast rate) and circulatory system (e.g. high blood pressure/hypertension), so trouble breathing, coughing, and collapse are potential symptoms as well.
To diagnose hyperthyroidism, complete blood count and chemistry panel will be performed to check all body system functions. Dehydration and liver changes are common and will be seen with this type of testing.
Thyroid tests will be run to check for high levels of T4 in the blood. If these tests are normal, but your vet still suspects hyperthyroidism, other thyroid hormone tests may be performed such as a T3 suppression test. Depending on how sick the pet is, other tests that can be run include:
X-rays and Ultrasound
Echocardiography (to evaluate the heart)
Blood Pressure Testing
Radioactive Scintigraphy (a type of specialized imaging used very rarely to look at the thyroid tissue itself)
Affected Breeds
In cats, the following breeds are at lower risk for hyperthyroidism than other cat breeds:
Most pets develop disease at older ages (average 12-12 years of age).
Treatment
Medications can be prescribed to control hyperthyroidism. Methimazole is used most commonly, usually twice daily. It primary comes in pill form, but veterinarians may be able to have it compounded into an oral liquid or a topical medication that is applied to the inside of the ear.
Once the medication is started, blood T4 levels will need to be checked periodically to make sure the pet is on the best dose. Thyroid medication will need to be given for the rest of the pet's life.
Prescription diets (e.g. Hill's y/d) are available for cats. Such diets work by decreasing the amount of iodine the cat eats. Iodine is a nutrient used by the thyroid gland to make T4, so decreasing it in the food can help control hyperthyroidism. Your veterinarian will be able to help you determine if prescription food will work for your cat. It isn't always effective, it can be somewhat expensive, and the food is not recommended for cats with certain other diseases (e.g. kidney disease).
Surgery to remove one or both thyroid gland lobes may be an option for some pets. Thyroid medication is usually started first to make sure the heart and circulatory system are healthy enough for surgery. Thyroid gland removal can be a tricky procedure, so the veterinarian may refer the pet to a specialized surgeon. Some pets will need to be on medication for hypothyroidism since the thyroid will not be producing thyroid hormones after the procedure. Very small glands called parathyroid glands, which control calcium levels in the blood, attach to the thyroid glands. Thyroid surgery may damage these glands or result in their removal (called hypoparathyroidism), so calcium medication may also be needed after surgery.
Radioactive iodine therapy is the best way to cure hyperthyroidism, but it can only be done at specialized veterinary institutions and is quite costly. Radioactive iodine targets only the diseased thyroid gland and avoids nearby structures, so it is safer and more effective than surgery. Often, healthy thyroid tissue remains undamaged, and the pet will not experience hypothyroidism or hypoparathyroidism side effects. Because the pet is given radioactive material, they will need to remain at the veterinary institution for days to weeks until the material has passed from their body.
Veterinary Cost
Cost varies depending on treatment:
Diagnosis Cost Ranges from $75-$350
Medication is $20-$50/ Month (depending on what is prescribed)
Prescription Food is $30-$90/ Month (depending how much and type of food)
Surgery Ranges from $750-$3,000
Radiation Therapy is $2,500-$3,000 for the Procedure (which may not include additional testing, boarding, etc.)
Recheck T4 Testing is Usually Around $50 (but will most likely only be needed 1-2 times a year once the right thyroid medication or food has been determined)
Prevention
Some veterinary scientists believe that certain things in the environment may play a role in causing hyperthyroidism. Chemicals in flame retardants (e.g. TDCIPP, PBDEs, PBBs) are currently being studied and may be connected to some cases of hyperthyroidism when pets are exposed to high levels.
Other possible factors that may play a role (primarily in cats) include:
Canned Food Diets
Certain Cat Litters
Primarily Fish-based Diets
Scientists are still doing a lot of research on these potential factors, so decreasing a pet's exposure to chemicals or avoiding certain litters/foods may help prevent hyperthyroidism, but it also may not. It is important to understand that in most cases, hyperthyroidism cannot be prevented, and its development in a pet is not anyone or anything's fault.
References
Mooney CT: Hyperthyroidism. Textbook of Veterinary Internal Medicine, 7th ed. St. Louis, Saunders Elsevier 2010 pp. 1761-79.
Ward CRL, Morgan RV: Diseases of the thyroid. Handbook of Small Animal Practice, 5th ed. St. Louis, Saunders Elsevier 2008 pp. 459-463.
Norsworthy GD, Carney HC, Ward CR, et al: 2016 AAFP Guidelines for the Management of Feline Hyperthyroidism. J Feline Med Surg 2016 Vol 18 (9) pp. 750.
Crossley VJ, Debnath A, Chang Y-M: Breed, Coat Color, and Hair Length as Risk Factors for Hyperthyroidism in Cats. J Vet Intern Med 2017 Vol 31 (4) pp. 1028-1034.
Wakeling J, Everard A, Brodbelt D, et al: Risk factors for feline hyperthyroidism in the UK. J Small Anim Pract 2009 Vol 50 (8) pp. 406-414.
Higgs P, Hibbert A: Managing hyperthyroidism in cats. Vet Rec 2012 Vol 171 (9) pp. 225-226.
Grossi G, Zoia A, Palagiano P, et al: Iodine-restricted food versus pharmacological therapy in the management of feline hyperthyroidism: A controlled trial in 34 cats. Open Vet J 2019 Vol 9 (3) pp. 196-204.Finch NC, Stallwood J, Tasker S, et al: Thyroid and renal function in cats following low-dose radioiodine (111Mbq) therapy. J Small Anim Pract 2019 Vol 60 (9) pp. 523-528.
Mensching DA, Slater M, Scott JW, et al: The feline thyroid gland: a model for endocrine disruption by polybrominated diphenyl ethers (PBDEs)? J Toxicol Environ Health 2012 Vol 75 (4) pp. 201-212.