| Hyperthyroid-Feline |
| by Susan Little DVM, Dipl ABVP (Feline) Hyperthyroidism (also called thyrotoxicosis) is one of the most common diseases of the middle-aged and older cat. It is a multi-system disorder caused by an increase in the amount of thyroid hormones (T3 and T4) produced by an enlarged thyroid gland. It was first documented in cats almost 30 years ago but the cause of the disease has been elusive. Although the enlargement in the thyroid gland is caused by a tumor called an adenoma, it is non-cancerous. The most common clinical signs of hyperthyroidism in cats include weight loss, increased appetite (although some patients have decreased appetite), vomiting, increased thirst and urination, hyperactivity, and diarrhea. The high levels of thyroid hormones can cause the development of heart disease, and these patients may have a heart murmur, difficulty breathing, high heart rate and arrhythmias. Veterinarians will order a blood chemistry panel as well as a thyroid hormone (T4) level in cats suspected of being affected by this disease. It is important to evaluate the health of the other major organs, including the kidneys and heart in these patients. Typically, hyperthyroid cats may have elevations in their liver enzymes. Chest x-rays and cardiac ultrasound may reveal secondary hypertrophic cardiomyopathy. Generally, the cardiac changes will reverse when the hyperthyroidism is treated. In some cases, specific heart medication may be needed to stabilize cardiovascular health. In recent years, it has been recognized that many hyperthyroid cats have concurrent chronic kidney failure that is being masked by the effects of hyperthyroidism. It has also been found that treatments directed at curing hyperthyroidism in these patients could lead to a worsening of their kidney function. Most hyperthyroid cats will have elevated levels of the thyroid hormone T4 in their bloodstream on a routine screening test. However, a small percentage of hyperthyroid cats will have normal T4 levels. If hyperthyroidism is still strongly suspected in these patients, a more sensitive test called the T3 suppression test can be performed to confirm the diagnosis. In this test, the cat is given seven oral doses of the thyroid hormone T3. Blood levels for both T3 and T4 hormones are checked before and at the end of the administration of the medication. In a normal cat, the administration of T3 hormone will cause the blood levels of T4 to drop by a negative feedback mechanism. In a hyperthyroid cat, the T4 levels will not decrease at all or will decrease very little. In this way, the veterinarian can distinguish between cats with hyperthyroidism and cats with other diseases with similar symptoms. Once hyperthyroidism has been confirmed, there are several treatment options. They include treatment with radioactive iodine, surgical removal of the gland, and treatment with anti-thyroid medications. The initial choice of treatment is often guided by concern about the patient's kidney function status. Some cats have detectable impairment of kidney function at the time of their diagnosis with hyperthyroidism, but many do not. It is difficult to assess kidney function accurately from routine blood testing in cats. Generally about 2/3 of the kidney function must be lost before routine blood tests will show any abnormalities. This has made it very difficult in the past to detect which cats with hyperthyroidism actually have concurrent kidney failure. However, Michigan State University has introduced a very sensitive test of kidney function in cats and dogs called the iohexol clearance test. In this test, a radiographic contrast agent called iohexol is injected intravenously and the rate at which the kidneys clear the agent from the bloodstream is measured. The test is carried out in the veterinarian's office and a series of blood samples is sent to the MSU lab for analysis. Since hyperthyroidism induces increases in blood pressure and blood supply to the kidneys, treating the disease will result in a drop in the blood supply to the kidneys. In a cat with kidney failure, this can cause a worsening of their kidney function in the few months after treatment for hyperthyroidism with either radioactive iodine or surgical removal of the gland. For this reason, patients with known kidney disease (either detected on routine blood work or with the iohexol clearance test) are often treated with anti-thyroid medications rather than surgery or radioactive iodine in an effort to preserve their remaining kidney function. Using medications allows the veterinarian better control over the concurrent kidney disease and may allow the patient to survive longer. Anti-thyroid medications in current use in North America include propylthiouracil (PTU) and methimazole (Tapazole®). Although they are both effective in decreasing thyroid hormone levels, PTU is associated with more adverse effects than methimazole. Methimazole is better tolerated and safer for long-term use in the cat. Approximately 15% of patients will suffer from side effects when taking methimazole. These may range from poor appetite, vomiting, lethargy and skin disease to more serious problems such as bone marrow depression and liver toxicity. In most cats, the adverse effects are mild and transient and do not interfere with continued treatment. Methimazole is now available from compounding pharmacies in a transdermal gel for those patients that are difficult to pill. Transdermal gels are applied to the inner ear and the medication is absorbed across the skin. Simple precautions must be taken to avoid inadvertent absorption of the drug by the person administering the medication. More recently, a drug called ipodate (Oragraffin®) has been used to treat hyperthyroidism in cats. Cats with severe hyperthyroidism do not respond to treatment with ipodate as well as cats with mild disease. A study at the Animal Medical Center of 12 cats being treated with ipodate showed no side effects. However, only eight of the 12 cats showed a positive response to the drug and the beneficial effects may not last for more than a few months. This drug is most useful for short-term management of cats requiring medical treatment in preparation for surgery. Unfortunately, ipodate is expensive, generally costing about $1/day, and is difficult to obtain. This is another reason why it may not be a suitable drug for long-term treatment. For hyperthyroid cats that are assessed with normal kidney function, surgery or radioactive iodine treatment are often recommended. Both these options provide a cure of the hyperthyroidism and avoid the need for life-long administration of medication. In areas where radioactive iodine treatment is available, it is usually the treatment of choice since this option avoids the risks of anesthesia and surgery. However, this is not a widely available treatment choice and veterinarians have become very skilled in surgical removal of the thyroid gland (thyroidectomy), making this an excellent option for treatment of hyperthyroidism in many cats. In general, the treatment a cat receives for hyperthyroidism will depend on individual status, including heart and kidney function. Concern about kidney failure is a major determinant of the course of treatment and may eliminate radioactive iodine or surgery as an option. The advent of new kidney function testing makes it possible to assess each patient's risk of kidney failure following treatment for hyperthyroidism. For further reading: Adams, W., G. Daniel, et al. Changes in renal function in cats following treatment of hyperthyroidism using 131-I. Vet Rad & Ultrasound 3 8(3): 23 1, 1997 Becker, T., T. Graves, et al. Effects of methimazole on renal function in cats with hyperthyroidism. J Amer Anim Hosp Assoc 36(3): 215-223, 2000 Broussard, J., M. Peterson, et al. Changes in clinical and laboratory findings in cats with hyperthyroidism from 1983-1993. J Amer Vet Med Assoc 206(3): 302, 1995 Graves TK. Hyperthyroidism and the kidney. In: August JR (ed) Consultations in Feline Internal Medicine 3, WB Saunders, Philadelphia; p. 345, 1997 Graves, TK. Feline hyperthyroidism: an update. Proceedings 9th annual Fred Scott Feline Symposium, Cornell University, 1997 Martin, K., M. Rossing, et al. Evaluation of dietary and environmental risk factors for hyperthyroidism in cats. J Amer Vet Med Assoc 217(6): 853, 2000 Mooney, C. Update on the medical management of hyperthyroidism. In: August JR (ed) Consultations in Feline Internal Medicine 3, WB Saunders, Philadelphia, p. 155, 1997 Murray LAS, Peterson ME. Ipodate treatment of hyperthyroidism in cats. J Amer Vet Med Assoc 211:63, 1997 Peterson ME, Randolph JR, Mooney CT. Endocrine diseases. In: Sherding RG (ed) The Cat: Diseases and Clinical Management, second edition, WB Saunders, Philadelphia; p. 1416, 1994 Peterson, M. and D. Becker Radioiodine treatment of 524 cats with hyperthyroidism. J Amer Vet Med Assoc 207(11): 1422, 1995 Last Updated: November 15 2002 -------------------------------------------------------------------------------- Please Note: The Winn Feline Foundation provides the feline health information on this site as a service to the public. Diagnosis and treatment of specific conditions should always be in consultation with one's own veterinarian. The Winn Feline Foundation disclaims all warranties and liability related to the veterinary advice and information provided on this site. -------------------------------------------------------------------------------- NOTE: The Winn Feline Foundation, organized primarily to fund health studies benefitting cats, unfortunately is not staffed to provide medical information or assistance on an individual basis. FYI, the Cornell University Feline Health Center operates a medical information "hotline" available to both practicing veterinarians and pet owners at 1-800-548-8937. Note that there may be a charge for consulting with them. Your own veterinarian is, of course, your best source for information. |