Hyperthyroidism in Cats
Hyperthyroidism is the most common endocrine disorder that affects cats. It creates a wide range of symptoms resulting from the overproduction of thyroid hormone. Recent advances in diagnostics and treatment have brought great improvements in the diagnosis and treatment of this disease. This disease was not commonly recognised prior to the late 1970s. The reason for the increase in prevalence is not known.
Why do cats get hyperthyroidism?
Cats get hyperthyroidism primarily through the development of an adenoma in the thyroid gland. The adenoma, a tumor, occurs in both glands in 70 % of the cases. The exact reason that cats develop these tumors is not currently known. There is speculation that there may be immunological, nutritional, and environmental factors involved. The tumor is primarily made up of thyroid cells and this increase in cells creates an increase in the secretion of thyroid hormone. A small percentage of the cancers are malignant.
Which cats are most likely to become hyperthyroid?
Hyperthyroidism occurs in middle to old-age cats with a reported range of onset between 4 and 22 years. The median age for acquiring the disorder is 13 years of age. Only 5% of hyperthyroid cats develop the disease before 10 years of age. There does not appear to be a breed or sex predilection.
What are the symptoms?
Thyroid hormone has an important role in controlling the body's metabolic rate, i.e. the general activity level, so cats with hyperthyroidism tend to burn up energy too rapidly and typically suffer weight loss despite having an increased appetite and increased food intake. The symptoms are wide and variable. The chart below lists the most common symptoms along with the incidence with which they are seen.
Symptoms % of Cats with Symptom
Weight loss 88%
Increased food consumption 49%
Vomiting 44%
Increased water consumption/ urination 36%
Increased activity 31%
Decreased appetite 16%
Diarrhea 15%
Decreased activity 12%
Weakness 12%
Labored breathing 10%
Panting 9%
Anorexia 7%
Heart murmur/ increased rate 55%
How is feline hyperthyroidism diagnosed?
Hyperthyroidism is diagnosed by measuring the level of thyroid hormone in the blood. There are two different thyroid hormones produced by the thyroid gland, thyroxine (T4) and triiodothyronine (T3). High T4 levels indicates hyperthyroidism in cats. Elevated T3 levels also indicate hyperthyroidism, however in 25% of the hyperthyroid cats T3 is not elevated even though T4 is high. For this reason the blood level of T4 is primarily used to diagnose hyperthyroidism. Sometimes an animal with concurrent renal, cardiac, or other debilitating disease may have hyperthyroidism but a normal or only slightly elevated T4. If an animal is suspected of having hypothyroidism but has a normal blood test it is suggested that the animal be re-tested after the current disease is under proper medical management.
Occasionally veterinarians will use other tests to confirm their diagnosis of hyperthyroidism. These include the T3 suppression test, the thyrotropin-releasing hormone stimulation test, and thyroid radionuclide uptake and imaging.
Diabetes, kidney disease, liver disease and other conditions that also affect older cats need to be ruled out prior to settling on a diagnosis of hyperthyroidism, even if tests indicate it is present.
How is feline hyperthyroidism treated?
There are currently three acceptable ways to treat hyperthyroidism in cats. They include surgical removal of the affected glands, treatment with anti-thyroid medication, and treatment with radioactive iodine. All of these treatments have their advantages and disadvantages and some are better than others for different age and disease presentations.
Antithyroid Medication: The use of oral antithyroid medication is still a very popular choice for the treatment of hyperthyroidism in the cat. The drug that is used is carbimazole (Neo-mercazol). Neo-mercazol is given in a pill form, commonly given at an initial dose of 5mg three times daily, until the thyroid hormone level falls to within the normal range. This usually takes two to three weeks, after which the dosing frequency can be reduced, usually to 5mg twice or even once daily. It works by blocking the synthesis of thyroid hormone in the gland. Some advantages to this product are that it doesn't cause the loss of function of the parathyroid gland that often occurs with surgery. The cost of the pill is relatively inexpensive and spread out over the life of the animal. If complications or problems develop it can be discontinued. It rarely causes hypothyroidism. Some disadvantages are that it doesn't cure the disease. It can be difficult to administer to some cats. Side effects of lethargy and vomiting occur in about 15% of cats that take cabimazole.
Surgery: Surgery involves the removal of the affected thyroid gland. Surgery removes the problem and can be an effective cure for this disease. It eliminates the need for daily administration of medications and the animal only has to be hospitalized for a couple of days. The surgery has a few drawbacks. One of these is the risk of removing the parathyroid gland, which is located next to the thyroid gland creating hypoparathyroidism. These glands control calcium regulation in the body and they are easily damaged during surgery. Death can result if calcium levels drop sufficiently. Therefore, calcium levels should be carefully monitored for a week if both of the thyroid glands are affected. If hypocalcaemia does occur it usually develops within the first few days following the surgery. Clinical signs include muscle twitches and weakness which can progress rapidly to convulsive seizures. Treatment is with supplemental calcium by intravenous injection and then by mouth. Additional treatment with activated vitamin D3 is also beneficial to allow this calcium to be used effectively. Most cats recover normal parathyroid function after a few days or weeks, but in severe cases treatment may need to be maintained for a few months before normal calcium regulation returns. If both glands are removed a hypothyroid state can develop.
Anaesthesia for thyroidectomy can be problematic. To reduce hyperthyroid-related anaesthetic complications patients should be pre-treated with anti-thyroid drugs to control their thyroid hormome level for three to four weeks before surgery. Any associated heart disease must also be treated where necessary.
Some veterinarians don't perform this surgery and a referral may be necessary.
Radioactive Iodine: Radioactive iodine therapy provides a simple, effective, and safe treatment for cats with hyperthyroidism. The basic principle behind treatment of hyperthyroidism with radioiodine is that thyroid cells do not differentiate between stable and radioactive iodine; therefore radioiodine is concentrated by the thyroid gland after intravenous administration. In cats with hyperthyroidism the radioiodine is concentrated in the hyperplastic tumor cells where it irradiates and destroys the hyperfunctioning tissue. The healthy thyroid tissue and parathyroid gland are not damaged by this treatment. A single subcutaneous injection of I131 is curative in around 95% of cases and in those cats where hyperthyroidism persists the treatment can be repeated. Some disadvantages to this treatment are that it can be expensive initially. All Creatures does not offer this treatment, so pets must travel to Sydney or Melbourne for the procedure. A third problem is that because of the radioactive material used, the animal needs to stay at the clinic until it clears the material from its body, usually around a week. Despite the extra initial effort and cost that this procedure requires, it would be my first treatment choice if one of my own cats develops hyperthyroidism. The results and safety appear to be excellent.
Radioactive iodine therapy is probably the best combination of safety and efficiency for treatment of hyperthyroidism. It is effective in about 90% of cats, no surgery or anesthesia is required and the parathyroid glands are not affected. For patients in which the daily administration of pills is undesirable this is the best method of treatment. The major disadvantage to this therapy is the required isolation of the cat at a treatment facility for 7 to 14 days following the administration of the radioactive iodine, due to safety concerns.
In older cats, this disease is common enough that routine screening is considered to be necessary by many veterinarians. Due to the potential for numerous secondary complications, such as heart disease and digestive problems, early diagnosis is a good idea. Consider asking your vet about tests for hyperthyroidism if your cat is over 10 years of age.
Author: Dr S Armstrong