Hyperthyroidism is the condition caused by excessive functional activity of the thyroid gland, resulting in excessive secretion of the thyroid hormones, thyroxine and triiodothyronine. It is the most common endocrine disease in domestic cats, and is extremely rare in dogs.
How Hyperthyroidism Affects Cats
Cats with this disease show a number of classic signs, including increased thirst and corresponding increased urination, weight loss despite increased appetite and food consumption, restlessness, hyperactivity, possible aggression, respiratory difficulty, elevated respiratory and heart rates and cardiac arrhythmias. They also tend to have an unkempt appearance and poor haircoat. A small percentage of hyperthyroid cats show weakness, lethargy, depression and anorexia. Affected cats typically have bilaterally enlarged thyroid gland lobes, although occasionally only one lobe is affected.
This disease predominantly affects older cats. There is no sex predilection. Purebred cats are much less likely to be affected than are mixed breeds.
Causes of Hyperthyroidism in Cats
Feline hyperthyroidism is the clinical condition caused by continual excessive secretion of thyroid hormones. There is no known genetic predisposition to developing this disease. Usually, the increased production of thyroid hormones is due to either a benign but functional thyroid tumor or autonomous enlargement of the thyroid gland. Most cases of hyperthyroidism in cats are caused by the increase in size of either one or both lobes of the thyroid gland, which continue to function and produce abnormally large amounts of thyroid hormones. This enlargement is caused by excessive cell division within the thyroid tissues; the reason for this hyperplasia is not well understood. Cancer is a much less common cause of hyperthyroidism in cats, but is more so in dogs. Thyroid hormones are responsible for moderating vital bodily functions, such as metabolic rate, protein synthesis and overall cell health. Excessive levels of these hormones disrupt these functions and cause the symptoms associated with hyperthyroidism.
Preventing Feline Hyperthyroidism
There is no realistic way to "prevent" hyperthyroidism in cats.
The prognosis for hyperthyroid cats with uncomplicated disease is excellent in the short-term and good in the long-term, as long as there is consistent owner compliance with medical treatment protocols. Radioiodine treatment and thyroidectomy are extremely successful in most cases. Cats with underlying renal disease have a more guarded prognosis, since both diseases are systemic and progressive. Kidney failure is the most common cause of death in hyperthyroid cats.
Hyperthyroidism is the condition of elevated and sustained metabolism caused by excessive amounts of circulating thyroid hormones. Affected cats normally exhibit a number of classic clinical signs, which usually are mild in the beginning and then increase in severity as the disease progresses.
Symptoms of Feline Hyperthyroidism
Cats with hyperthyroidism have abnormally high metabolism, which leads to a series of multisystemic clinical signs.
Increased Appetite with Weight Loss
One of the classic consequences of the increased metabolic rate caused by excessive circulating thyroid hormones is an increased appetite, which in turn causes increased consumption of food if it is available (called "polyphagia"). Oddly, despite this marked increase in appetite and presumably increased food intake, affected cats lose weight. Cat owners usually are baffled by their pet's increased appetite, increased food intake and corresponding weigh loss. How can their cat eat so much but still lose weight? The answer is that hyperthyroidism dramatically increases the cat's system-wide metabolism, which is what causes the high hunger with weight loss. The increased metabolic rate can also cause the cat to become much more active, or restless, especially in the early stages of the disease.
Increased Water Consumption
Cats with hyperthyroidism also become more thirst than normal and tend to drink an unusually large amount of water (called "polydipsia). As a result, they will urinate more frequently and in larger volume. Owners notice more frequent visits to the litter box, and the need to clean it more often than usual.
Affected cats frequently have gastrointestinal symptoms, including vomiting and diarrhea.
Hyperthyroid cats often have difficulty breathing (dyspnea) and have an elevated respiratory rate (tachypnea).
Enlarged Thyroid Gland
The vast majority of cats with hyperthyroidism have palpably enlarged thyroid glands bilaterally (on both sides). Sometimes, owners can feel these enlarged areas each side of the trachea, under and behind the lower jaw along the upper throat. As these lobes of the thyroid become enlarged, and the cat loses weight, they become increasingly prominent.
Behavioral and Other Signs
Most affected animals become hyperactive, display excessive nervousness and/or become aggressive. Much less commonly, they may act weak and lethargic. Typically, they present in poor overall body condition, with an unkempt haircoat and sometimes thickened nails. They may become intolerant of heat and seek out cool places. On physical examination, the veterinarian often will find a racing heart rate (tachycardia), cardiac arrhythmias and/or murmurs, irregularities in the retina and dehydration.
The clinical signs of hyperthyroidism in cats can mimic those of kidney disease, liver disease, diabetes and cancer. Diagnosis is based upon a thorough physical examination and history, the cat's presenting signs, a complete blood panel and blood tests to detect the levels of circulating thyroid hormones. In some cases, imaging techniques are helpful as well.
Diagnosing Hyperthyroidism in Cats
Physical Examination and History
Hyperthyroidism causes a classic series of signs in cats. Most veterinarians begin their diagnostic journey by getting a complete description from the owner of what unusual clinical signs they have noticed in their cat, followed by a thorough physical examination. Typically, the doctor will be able to feel (palpate) enlarged lobes of the thyroid gland which are located on each side of the cat's trachea.
A complete blood work-up, consisting of a complete blood count and serum biochemical profile, together with a urinalysis, are usually a standard part of the diagnostic process for cats that seem to have some systemic disease. Additional blood tests are available to rule in or rule out hyperthyroidism. The advanced test that normally is done first in cats suspected of having hyperthyroidism is called a serum total T4 concentration test. This measures the total amount of circulating thyroxine hormone in the blood and often is diagnostic of hyperthyroidism if it is consistently elevated. Other blood tests are available to measure the amount of other thyroid hormones in circulation. Taken together, the results of these tests will help the veterinarian make a definitive diagnosis. Unfortunately, some cats with mild hyperthyroidism have normal blood test results, and in these cases additional testing may be necessary.
Thoracic radiographs, and possibly an electrocardiogram and/or echocardiogram, are frequently part of the diagnostic work-up in cats suspected of having hyperthyroidism – mainly to assess heart size, the presence and cause of cardiac arrhythmias and generally to assess the severity of heart disease. In some instances, thyroid imaging is used in the diagnosis of hyperthyroid cats. This technique, called thyroid gland scintigraphy, is only available at certain specialized veterinary facilities and usually is only performed in cases where the hyperthyroidism is thought to be caused by cancer, or in cases where the attending veterinarian deems it valuable to visualize how much of thyroid gland has been affected by the disease.
The goals of treating feline hyperthyroidism are to return serum thyroid hormone levels to normal if possible and to eliminate the clinical signs of disease. This more specifically means increasing the cat's weight and overall body condition and maximizing its quality of life. Depending upon the exact cause of the disease, treatments may consist of medication, surgery and/or or radioactive iodine therapies. Since hyperthyroidism is not an acute disease, emergency therapies are rarely necessary.
Treating Hyperthyroidism in Cats
By far the most common treatment for feline hyperthyroidism is lifelong daily administration of oral medication that reduces the function of the thyroid gland and thus reduces secretion of thyroid hormones. Frequently, once cats start on this course of therapy, underlying renal problems can surface. Your veterinarian will watch for this during the early course of medical treatment.
If the hyperthyroid condition is the result of a tumor on one or both lobes of the thyroid gland, surgical removal of the gland (called a thyroidectomy) is possible and can possibly be curative. If only one lobe is affected, the cat may not require further treatment or medication, although it is possible that the remaining lobe will eventually become hyperactive. If both lobes need to be and are removed, the cat will require oral thyroid hormone replacement therapy for the rest of its life.
Radioactive Iodine Therapy
Radioactive iodine therapy is considered to be a safe and highly effective form of treatment for feline hyperthyroidism. In fact, it is considered to be the best therapeutic option for long-term control of this disease in cats, as long as the kidneys are not damaged. This treatment destroys all thyroid tissue. The availability of veterinary facilities that offer this treatment is somewhat limited, but is increasing. The protocol requires special handling facilities and post-therapy isolation for several days to weeks due to radioactivity.