Hyperthyroidism is the most common hormone imbalance in cats. This disease emerged in the 1980s but has become so common that a wealth of information is available. It is a common disease in cats, and mostly afflicts cats middle-aged and older. Also called thyrotoxicosis, hyperthyroidism is caused by an increase in production of thyroid hormones (known as T3 and T4) from an enlarged thyroid gland in a cat’s neck.
What Causes Hyperthyroidism?
Hyperthyroidism is caused by a benign growth in the thyroid gland that is over-producing T4. It is important to realize that these tumors are almost always benign and represent a form of goiter rather than a form of cancer. Less than three to five percent of hyperthyroid cats have a cancerous thyroid growth.
Purebred cats, especially the Siamese and Himalayan breeds, appear to have a decreased incidence of hyperthyroidism (meaning they are less likely to develop this condition). This implies that there are genetic factors at work as well.
What is Thyroid Hormone?
Active thyroid hormone (nicknamed T3, short for triiodothyronine) sets the body’s metabolic rate, sort of like a volume dial. One might say T3 determines how hard or how fast each cell works to do its job. Every cell of the body is affected by T3.
The thyroid glands (there’s one gland on each side of the windpipe) do not produce T3. Instead, they produce an inactive form called T4. Tissues of the body absorb T4 and convert it to T3. Doctors will readily make references to T3 and T4. It is a good idea to know what they are referring to. While T3 is the active hormone, it turns out that more meaningful information is gained by measuring T4.
The thyroid gland is found in your cat’s neck and uses dietary iodine to make thyroid hormones that help regulate important body functions. Thyroid hormones affect nearly all the organs in the body; therefore, thyroid disease often causes secondary problems iincluding your cat’s:
- Body temperature
- Blood pressure
- Heart rate
- Gastrointestinal (bowel) function
Symptoms of Hyperthyroidism
There are many clinical signs associated with feline hyperthyroidism. The hallmark sign is weight loss despite excellent appetite.
Hyperthyroid cats are also extra thirsty and are sometimes brought to the vet because they are drinking so much water. They are commonly restless and especially demanding of attention and/or food. Many cats have chronic intermittent vomiting or diarrhea and may be urinating in inappropriate locations.
In a normal cat, the lobes of the thyroid gland cannot be felt with your fingers. In the hyperthyroid cat, at least one lobe is usually prominent and may be detected by your veterinarian during a physical exam.
Cats afflicted with hyperthyroidism usually develop a variety of signs that may be subtle at first but that become more severe as the disease progresses. Signs of hyperthyroidism can vary in severity depending on how long a cat has been ill. Common signs are:
- weight loss – most common sign
- increased appetite – caused by increased energy expenditures, increased metabolic rate, decreased work efficiency causes a ravenous appetite and increased food intake. It is not unusual for an owner to say, “I don’t understand why she is losing weight, she eats all the time”.
- Behavior changes
- increased activity and restlessness – caused by increased metabolic rate – almost like the cat is “on speed”. Elevated levels of T4 affect the nervous system. You may see vocalization, restlessness and agitation.
- aggressive or “cranky” behavior – also caused by increased metabolic rate
- a poor hair coat – this can be a patchy, matted haircoat with excessive shedding. They tend to look scraggly and have poor grooming habits.
- Heat intolerance – caused by loss of body fat and muscle wasting/fatigue.
- a fast heart rate – is related to increased metabolism and changes to the heart muscle (hypertrophic cardiomyopathy – thickening of the heart walls) due to overwork and inefficiency of blood movement through the heart and lungs.
- Hypertension – related to heart changes and increased metabolism
- occasionally difficulty breathing – panting and respiratory distress can be due to respiratory muscle weakness and increased carbon dioxide production. Stress can cause life-threatening conditions in both the heart and lungs. Your veterinarian will try to minimize the stress of your cat’s physical exam.
- increased water drinking – caused by an increase in production of metabolic wastes and dilute urine.
- increased urination – often caused by a condition called medullary washout when there is increased blood flow to the kidneys causing loss of important electrolytes, proteins, and minerals needed for the kidneys to concentrate the urine. Often there is kidney disease that goes along with hyperthyroidism in the later stages of the disease
- periodic vomiting – from the effects of T4 on the vomiting stimulus to the nervous system and due to distension of the stomach from over-eating.
- increased amount of stool or diarrhea – from overeating and hypermotility (increased movement of the intestine). Often there is a malabsorption and maldigestion syndrome associated with these changes in the gastrointestinal tract that can cause diarrhea. But with overeating the food in being consumed faster than it can be digested and causes an upset in the bacteria in the intestines.
- occasionally weakness – may be caused by generalized weakness in the muscles and muscle wasting.
- occasionally depression – also caused by muscle weakness and muscle wasting.
- Neurologic changes due to thiamine deficiency, potassium deficiency, and other vitamin and mineral deficiencies.
- Small kidneys due to concurrent kidney disease from hypertension and increased metabolism
How is Hyperthyroidism diagnosed?
A veterinarian who suspects a cat has a thyroid problem will conduct a physical examination and palpate the cat’s neck area to check for an enlarged thyroid gland. The cat’s heart rate and blood pressure may also be checked. If thyroid disease is a possibility, your veterinarian will likely order lab work to check for other geriatric or concurrent diseases. Lab tests may include:
- Complete Blood Cell Count (CBC) – to look for changes in white blood cell counts to check for infection, anemia, dehydration, or changes that indicate on of the other geriatric diseases.
- Biochemistry profile (Chem 14)
- to look for elevated liver enzymes which are common in hyperthyroid cats. These elevated values could be related to the increased metabolism, malnutrition, infection, congestive heart failure or direct toxic effects of the elevated T4 hormone.
- Hyperglycemia to rule out diabetes mellitus
- Elevated kidney values to look for dehydration, kidney insufficiency, or kidney failure.
- Elevated phosphorus (hyperposphatemia) with concurrent low calcium (hypocalcemia). The elevated T4 causes skeletal bone resorption which causes changes in the minerals and possible uremic gastritis which can lead to ulcerated stomachs and intestines.
- Thyroid profile (T4) – elevated T4 levels is a definitive diagnosis of hyperthyroidism. Most cats with hyperthyroidism have elevated levels of the thyroid hormone T4 in their bloodstream, but a small percentage of cats with hyperthyroidism have T4 levels within the normal range. If your cat does not have elevated T4 levels but your veterinarian still suspects your cat has hyperthyroidism, additional tests may be recommended.
- Radionuclide imaging – allows the visualization of all thyroid tissue. Hyperthyroid cats will have an increased uptake in the thyroid glands making them easier to measure size and function. If the diagnosis is not obvious by blood tests, a nuclear medicine scan of the thyroid glands can be performed at certain specialty veterinary practices. The cat is given a small dose of a radioactive compound that travels by the blood to the thyroid glands. Hyperactive thyroid glands accumulate more of the compound than normal glands. After this test the cat must be hospitalized for a few days while it clears the radioactive compound from its body.
- Urinalysis and/or urine culture and sensitivity – may be done to rule out a kidney or bladder infection that may be artificially decreasing the thyroid level in a hyperthyroid cat. This is called “sick” euthyroid syndrome.
- ECG (electrocardiogram) – High levels of thyroid hormone may cause heart disease. The heart may appear enlarged on x-ray or ultrasound and may show abnormal electrical activity on an ECG (electrocardiogram). Heart disease may cause fluid to build up in or around the lungs. Cats with serious heart disease and hyperthyroidism need to be treated for both diseases. The heart disease will reverse in many cats after successful treatment of hyperthyroidism.
How is hyperthyroidism treated?
There are three types of treatment for hyperthyroidism:
- Lifelong oral anti-thyroid medications
- surgical removal of affected thyroid glands
- treatment with radioactive iodine
All three treatments will reduce thyroid hormone levels and the signs of hyperthyroidism. Discuss the 3 options with your veterinarian. If your pet has other diseases, one treatment may be better for your cat than another.
- The anti-thyroid pill for cats is Felimazole (methimazole).
- This drug is given once up to three times daily and must be continued for life. It takes several weeks for these medications to reduce blood thyroid hormone levels to normal. If they are discontinued, thyroid hormone levels will return to high levels over a few weeks.
- Felimazole may be used to reduce thyroid hormone levels to normal before surgically removing the thyroid gland(s).
- Cats with heart disease may be too sick and fragile to anesthetize for surgery in which case felimazole can be given until the heart improves and the cat is stronger.
- Felimazole or Methimazole may produce side effects in cats including depression, vomiting and lack of appetite. These signs usually resolve without stopping the medication. A more serious side effect is the development of low blood cell counts which are more likely to develop during the first 3 months of treatment. Blood cell counts should be evaluated every 2 weeks during the first 3 months. If blood cell counts decrease methimazole is stopped and another treatment method should be considered.
- Frequency of administration is determined by your veterinarian and usually based on T4 blood levels.
- Methimazole Transdermal Gel.
- This is a drug that is created at a compounding pharmacy. It is applied to bare skin once or twice a day. Frequency of administration is determined by your veterinarian and usually based on T4 blood levels.
- Enlarged thyroid glands can be surgically removed.
- Methimazole is given for 1 to 2 months before surgery so that thyroid hormone levels are normal at the time of surgery. If both glands are enlarged, they can both be removed, and most cats will still produce enough thyroid hormone by a few thyroid cells scattered throughout the body to prevent hypothyroidism (abnormally low thyroid hormone levels).
- A few cats will become hypothyroid and may need to take thyroid pills. Surgical removal of the thyroid gland(s) can usually be performed without complications.
- Occasionally complications may develop including damage to the parathyroid glands, which are closely attached to the thyroid gland, damage to nerves close to the thyroid gland or damage to the voice box. Parathyroid gland damage causes low blood calcium that may cause seizures. Low blood calcium is treated with calcium or vitamin D. Nerve damage causes abnormal size of the pupils of the eyes and droopy eyelids. Damage to the voice box causes a change in voice.
- Some cats will remain hyperthyroid after surgical removal of the thyroid glands. These cats have thyroid cells in abnormal locations, including inside the chest cavity where surgical removal is difficult. This extra thyroid tissue is called ectopic thyroid.
- Cats that have had surgery may have recurrence of hyperthyroidism. Blood thyroid hormone levels should be measured once or twice a year.
- Treatment with radioactive iodine is performed at selected specialty veterinary practices. Your veterinarian will refer you to one close by.
- Radioactive iodine is given intravenously and will accumulate in the abnormal thyroid tissue killing the abnormal thyroid cells but sparing the normal thyroid cells. Radioactive iodine will also accumulate in ectopic thyroid tissue.
- Radioactive iodine treatment is very effective and rarely causes hypothyroidism. The cats do not have to be placed under anesthesia for the procedure.
- The disadvantages of radioactive iodine treatment include the need to travel to a facility that offers this treatment and the need for the cat to remain hospitalized until the level of radioactivity decreases to a safe level as determined by the state radiation control office (usually 1 to 3 weeks).
- Another disadvantage of this type of treatment is cost. However, the cost of radioactive iodine treatment is less expensive when compared to the cost and frustration for the owner for a lifetime oral medication.
- 80% of cats will be cured with this method.
- Diet – 20% of cats with hyperthyroidism may be controlled with diet alone. The most common diet used is Hills y/d. This diet is lower in salt and iodine and limits the amount of thyroid hormone that is produced. The importance of nutrition in the treatment of this disease is very important. The food your cat eats plays an important role in her overall health and well-being. Balanced nutrition is an essential part of an active, healthy lifestyle.
Why is it so Important to Treat Hyperthyroid Cats?
Hyperthyroid cats frequently experience reduced quality of life through weight loss, muscle deterioration, chronic vomiting or chronic diarrhea.
Hyperthyroidism causes severe organ changes and diseases. Heart disease, hypertension (high blood pressure), kidney disease, rapid weight loss, neurologic problems are common diseases seen in cats. Not all cats experience these signs at the time of diagnosis but they can be present as invisible diseases resulting in heart failure, kidney failure, liver disease, sudden blindness, or sudden death and all can be prevented with timely treatment for thyroid disease.
Monitoring the disease
It is important to monitor cats with hyperthyroidism. We want to be sure we have the disease controlled and monitor for any other diseases that may develop.
During the first couple of months your cat will need to have the thyroid hormone levels monitored with a T4 test every 3 – 4 weeks until the levels are back into the normal range. A CBC may also be needed to monitor for adverse effects of the thyroid treatment. A chemistry panel (Chem 14) may also be needed to monitor for the appearance of other diseases that may occur.
Once the hyperthyroidism is controlled (or cured depending on treatments performed – see discussion above on treatment options), your cat will need to have regular bloodwork to monitor for changes in organ function and adverse effects of the medications. Blood work may be required every 3 months or every 6 months depending on the severity of the disease and any additional diseases that may be present.
Blood pressure should be performed at every veterinary visit to monitor the progression of the disease and its effects on other organs in the body.
An ECG may be needed every 6 months based on the severity of the heart disease, if present.
The three therapies for the treatment of hyperthyroidism are usually successful. The prognosis for this disease depends on type of treatment selected, progression of the disease when first diagnosed and any underlying diseases that may be present. The overall prognosis tends to be good.