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Hyperthyroidism in Cats

The thyroid is a two-lobed gland located in the neck of people, dogs, cats, and other animals. One lobe is on each side of the trachea (windpipe). The thyroid produces thyroid hormone, a substance that is transported via the blood to all cells in the body.

Hyperthyroidism is a serious and relatively common endocrine disorder resulting from excess in thyroid hormone in the circulation. Most commonly, this condition is caused by a functional (hormone producing) enlargement of the thyroid gland(s) or a hormone producing thyroid tumor. Fortunately, the more serious thyroid carcinoma rarely produces this condition in cats. In about 80% of cats, both thyroid glands will be enlarged and/or contain tumor tissue. Fifteen percent will have one-sided involvement. As to how or why these enlargements or tumors appear is uncertain, but this condition has been seen more frequently in the last two decades

Because thyroid hormone exerts a profound effect upon the metabolic rate of all cells in the body, cats with this condition are literally running at "top speed" all the time, which is ultimately quite detrimental to the health of the cat. 

The primary function of thyroid hormone is control of the rate that cells function:

  • Too much thyroid hormone makes cells work too fast. Low thyroid function (hypothyroidism) is relatively common in dogs and quite rare in cats.
  • Too little thyroid hormone causes cells to work too slowly. Excess thyroid function (hyperthyroidism) is rare in dogs but is one of the more common diseases diagnosed in cats that are 8 years of age or older.

It is not known exactly why cats develop hyperthyroidism:

  • About 15% of hyperthyroid cats have a single thyroid tumor in one of the two lobes, called an adenoma (not a cancerous tumor) that produces too much thyroid hormone.
  • About 80% of hyperthyroid cats have abnormalities in both thyroid lobes with both lobes producing excess thyroid hormone. These cats also have a benign (noncancerous) condition called adenomatous hyperplasia.
  • Only about 3% of hyperthyroid cats have a malignant (cancerous) thyroid tumor.

Symptoms of hyperthyroidism

Each cat responds to his or her hyperthyroidism a little differently, causing observed abnormalities to vary from cat to cat. Among the most common owner observations are the following:

  • Weight loss
  • Increase in appetite
  • Patchy hair loss or failure to groom (some cats have been observed to pull their hair out)
  • Increase in water intake
  • Increase in urine output
  • Increase in activity level (some cats are persistently restless or nervous)

Some hyperthyroid cats have a change in behavior and when this happens, they tend to become irritable, mean, or easily upset. Vomiting and diarrhea are a little less common. Some hyperthyroid cats have unusually bulky stools and others have unusually large amounts of stool. Relatively uncommon but well-documented problems caused by hyperthyroidism include panting (open mouth breathing), difficulty breathing, loss of appetite, muscle weakness, listlessness, and seeking cool places.


The diagnosis of hyperthyroidism, regardless of the underlying changes within the thyroid gland itself, is relatively easy in most cats. Most hyperthyroid cats have too much thyroid hormone (thyroxine, or T4) in their blood all the time. This can be confirmed with a simple and relatively inexpensive blood test measuring T4. A small percentage of hyperthyroid cats do not have a “diagnostic” blood T4 concentration and, in this situation, your veterinarian may wish to repeat the test a few days, weeks, or months later.

Your veterinarian may elect to use a different thyroid blood test (called the “free T4 test”) or their recommendation may be to have a thyroid scan performed on your cat. Free T4 by equilibrium dialysis (fT4ED) is the most accurate way to measure fT4 and is more sensitive in assessing thyroid function in sick animals where the T4 may be depressed. fT4ED is less affected by altered protein levels as the samples are dialysed prior to assay. All these options are excellent and each one tends to complement the others. Virtually any veterinary lab can run the T4, free T4, and free T4 by equilibrium dialysis assays. Thyroid scans, however, require special facilities and usually require referral to a hospital with this equipment. Regardless, if your veterinarian believes your cat has this condition, the diagnosis is usually straightforward and relatively inexpensive.

Because hyperthyroid cats tend to be older, and because they tend to have worrisome symptoms consistent with various non-thyroid-related diseases, and knowing that excesses in thyroid hormone can have deleterious effects on various organ systems, your veterinarian will likely recommend tests in addition to those which identify the thyroid status of your cat.

Some of these additional tests may include the following:

  • CBC (complete blood count), to assess the red and white blood cells
  • Chemistry panel and UA (urinalysis), to assess the function of various organs
  • X-rays of the chest, to rule out congestive heart failure (a possible complication of hyperthyroidism) and other thoracic problems (not associated with the disease)
  • X-rays or ultrasonography of the abdomen, to assess the various abdominal organs.



Treatment, no treatment, and your cat's kidneys. Hyperthyroid cats that are not treated usually become progressively more ill. They tend to be quite thin, weak, and are likely to develop life-threatening problems.

 If your cat is not symptomatic, there should be no rush to treatment. In contrast, delaying treatment for symptomatic cats is unreasonable. Three commonly used treatments are available for managing hyperthyroidism in cats. Each treatment has the potential to provide excellent results and each has both advantages and disadvantages. Your veterinarian will explain the choices to you and help you decide which option may be best for you and your cat. Treating your cat will usually either return it to a reasonable state of good health or permanently cure the cat of this disease.

 One of the most difficult issues in the treatment of hyperthyroidism is the unmasking of preexisting renal disease. Hyperthyroidism results in increased glomerular filtration rate and increased renal plasma flow, as well as increased cardiac output and blood pressure. These physiologic changes, although detrimental to long-term renal health, may augment renal function in some hyperthyroid cats to the extent that their true renal status is obscured. 

Successful treatment of hyperthyroidism by any means and a return to the euthyroid state result in decreased glomerular filtration rate, leading to azotemia in some cats This decline in function represents an unmasking of the cat's true renal function, rather than nephrotoxic effects of treatment. Therefore, a reversible method of therapy (antithyroid medication) should be used as the initial treatment in newly diagnosed cats; thus, if their renal function declines to an unacceptable level, treatment can be discontinued or decreased so that the cat is left mildly hyperthyroid.


Methimazole (TapazoleŽ, Lilly) is an oral (pill) medication that works by blocking the production of thyroid hormone. The effects of this drug are completely reversible. If you stop giving this drug, most cats quickly return to the same thyroid condition they had prior to using the medication. Thus if correcting the hyperthyroidism causes kidney results to become worrisome, then the medication can be discontinued or tailored to a dose that may not completely resolve the hyperthyroidism but also may not harm the kidneys. The drug is readily available and not terribly expensive. The major attribute of this drug is its ability to consistently decrease thyroid function. It is effective in virtually every cat. The drug usually works best when given twice daily.

Because this medication has potential side effects, starting out with an extremely low dose and slowly increasing the dose to effect is safest and most effective. Some veterinarians start with 2.5 once daily for 2 weeks, a dose that is too low to resolve the hyperthyroidism but one that rarely causes side effects. However, just getting a little drug into cats tends to minimize side effects as the dose is increased to achieve desired blood test and clinical responses. Therefore, as needed, they increase first to 2.5 mg twice daily for two weeks, check the blood, and then increase by 2.5 mg per day at 2-week intervals.

The major negative aspect of methimazole is that some cats are not the best pill takers. Rather than fight these cats, methimazole can be “compounded” by some pharmacists into a topical cream. The owner then uses a latex glove or finger cover, places the correct dose on his or her finger, and rubs the medication (usually in a tiny amount of cream) into the inner aspect of one ear. The medication needs to be rubbed in well (usually for 30 to 120 seconds) and then about 20 minutes later any excess cream (if there is any left) should be cleaned away with cotton and warm water.

 Medication compounded by a good pharmacist is at least as good; frequently more effective in this route than it is when given orally. Because it is sometimes more effective, some veterinarians use a lower dose to start: usually 1 or 2 mg once daily and then increased as needed.

Other negatives about methimazole, especially when given orally, are that it causes some cats to lose their appetites or vomit. These side effects are much less common in cats given topical rather than oral medication. Even less common, but more worrisome, side effects include liver damage and decreases in red blood cells, white blood cells, and platelets. Platelets help blood to clot so loss of platelets can cause bleeding, whereas loss of white blood cells predisposes these cats to infection, and loss of red cells is anemia that can make a cat weak or die. Rarely, treated cats develop a type of reaction and they begin to scratch their faces uncontrollably.

 Although many of these side effects are alarming, they are not common. Therefore this drug is given to virtually every hyperthyroid cat. It is either their only means of treatment or it is given to test the effects of resolving the increased thyroid hormone concentrations on kidney function prior to using a permanent form of therapy.


 Neo-MercazoleŽ (Nicholas Laboratories Ltd., Slough, England) is an effective alternative treatment if your cat does not tolerate methimazole well.

Carbimazole is a pro-drug that is converted to methimazole in the body. A recent study of the efficacy and safety of carbimazole in the treatment of hyperthyroidism in cats suggests that it may be associated with less adverse effects than  methimazole. In that report, only 4% of cats developed hematologic abnormalities (lymphocytosis and leukopenia). These abnormalities were very mild and cessation of carbimazole therapy was not necessary.

In contrast to methimazole treatment, twice-daily treatment schedules may be inadequate during the initial weeks of administration; an initial dosage of 5-mg tid is recommended initially. Euthyroidism is typically attained within 2 weeks of the start of therapy. Long-term, twice-daily schedules are effective in controlling hyperthyroidism. Adverse reactions are similar to those seen in cats receiving methimazole, but occur less frequently. Cats being treated with carbimazole should be monitored in the same manner as that suggested for cats receiving methimazole.  Carbimazole is not available commercially in the United States due to a marketing agreement with Lilly (Tapazole). However, carbimazole is available through compounding pharmacies. (e.g., Island Pharmacy Services, 1707 Highway 51 N., Woodruff, Wisconsin 54568, 1-800-328-7060).


Surgery can be used to remove the abnormal thyroid lobe or lobes. Most veterinarians either do this surgery or can refer you to a colleague who does it. The negatives associated with this surgery are that it does require anesthesia. Many cats suffer from heart, liver and intestinal changes associated with hyperthyroidism, which make such patients more of a surgical/anesthetic risk.   If both glands are involved, it is possible to remove them both, but exacting and special attention must be given to try and save at least one of the four parathyroid glands which are closely associated with the thyroid glands. This can be tricky, especially to maintain the microscopic blood supply to these tiny organs.

If both glands were removed, there is always a question as to how well the parathyroid glands will function; if they fail the cat could have a fatal seizure due to a severe drop in blood calcium. Some cats never have complications, others will require supplements and vitamin D therapy to help them otherwise maintain their blood calcium levels. Regular post-surgical thyroid checks are also needed to guard against recurrence and/or see if there is a need for thyroid supplementation

The most common complication of surgical thyroidectomy is hypocalcemia as a result of inadvertent damage to the parathyroid glands or their blood supply resulting in hypoparathyroidism. Post-thyroidectomy hypocalcemia can be severe, necessitating immediate treatment. Severe clinical signs of hypocalcemia include tetany and generalized seizures. Serum calcium concentrations should be monitored postoperatively regardless of clinical signs. Should overt hypocalcemia develop, immediate treatment is essential. Emergency treatment of hypocalcemia entails intravenous administration of calcium gluconate.

.A 10% fatality rate has been associated with this surgery.


Radioactive iodine.

Radioiodine therapy is the treatment of choice for feline hyperthyroidism. It is the safest, least stressful, most specific treatment available, and has an excellent cure rate (>90%). It is a particularly useful treatment for cats with bilateral thyroid involvement (approximately 80 per cent of cats), cats with ectopic (noncervical) thyroid tissue, and the rare patient with thyroid carcinoma. No side-effects are seen from the radiation therapy per se, and the treatment is well-tolerated by even the most debilitated, elderly hyperthyroid cat. When patients are pretreated with thioureylene drugs and selected carefully, the risk of unmasking preexisting renal disease is minimum.

Radioiodine therapy is very specific because the thyroid glands are the only tissues in the body that concentrate plasma iodine actively. The glands cannot differentiate between normal dietary iodine (stable iodine) and radioactive iodine used for therapy. Therefore, radioiodine is concentrated by the active (adenomatous, or rarely, carcinomatous) thyroid tissue, which is destroyed. The normal thyroid tissue is atrophied in the hyperthyroid cat because of lack of stimulation by thyroid-stimulating hormone, does not accumulate radioiodine, and thus is spared. After radioiodine therapy, cats go through a period of subclinical hypothyroidism that is almost always asymptomatic and does not require therapy in the vast majority of cats. During this period, thyroid-stimulating hormone secretion resumes, the atrophied thyroid tissue becomes functional, and the cat then becomes euthyroid permanently.

 The benefits of this treatment include the following:

  • It is an extremely effective form of treatment
  • It resolves hyperthyroidism quickly
  • It requires no anesthesia
  • No pills are necessary
  • It almost never causes hypothyroidism

The negative aspects of this form of treatment are that sophisticated facilities licensed for use of radioactive materials are required. Although this treatment is not available everywhere, it is available in numerous sites throughout the United States and more treatment centers continue to open. It tends to be more expensive than surgery annually, in part because treated cats must remain hospitalized until their body levels of radioactivity are safe. This allows all urine and stool (which contain radioactivity) to be properly disposed. Hospitalization usually ranges from several days to as long as 2 weeks.

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