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,
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,
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

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