Hyperthyroidism Treatment Options

There are three methods of treatment for feline hyperthyroidism, each of which has different pros and cons. We have created summaries explaining each.




Thyroid Treatment: Radiotherapy

Imaging with Nuclear Medicine and Treatment by Radiotherapy

This method of therapy is generally considered the safest and most effective method of treatment for feline hyperthyroidism. First, the procedure involves a nuclear medicine scan in which the cat receives an injection of the radioactive compound pertechnetate. The resulting scan shows the location and size of the cat’s thyroid glands and confirms the disease. The scan also indicates whether one or both glands are involved and provides information needed to calculate the therapeutic dose of iodine 131 that will be used in treatment. An additional benefit of the scan is that it can identify the 3 to 5% of cats who have a malignant tumor and detects areas of tumor spread.

See more about malignant thyroid tumors.

After the pertechnetate scan is complete, malignancy has been ruled out, and hyperthyroidism has been confirmed, a radioactive isotope of iodine called iodine 131 can be used to destroy the abnormal thyroid tissue. Normally, iodine is joined to the amino acid tyrosine in the thyroid gland to create T4. Iodine 131 is carried directly to the thyroid gland as though it were regular iodine. Iodine 131, being radioactive, emits high speed electrons that kill the surrounding abnormal thyroid tissue. Because these electrons penetrate only fractions of an inch, only the thyroid gland experiences the radiation and the rest of the body is spared.

This treatment need not be repeated and no additional therapy is required; however, while humans receiving similar treatment are promptly allowed to go home post-treatment, the process is different with cats. The Nuclear Regulatory Commission requires cats to remain hospitalized in a special isolation ward until the iodine 131 is at sufficiently low levels. This timeframe amounts to about three or four days of hospitalization depending on how quickly radiation levels drop. After the cat comes home, some restrictions must be imposed for another week and a half or so: the cat must use special flushable litter, the cat is not allowed outside, the cat's daily direct contact time with the owner is limited, children and pregnant women may not have contact with the cat during this time, etc.

Blood work is monitored following treatment to ensure return to normal thyroid status. Occasionally, a single course of radiotherapy is inadequate and a second course is needed (see below).

Advantages of this Method of Treatment

  • Treatment is a one-time event (only 2 to 4% of cats require a second treatment) and no on-going therapy is required.
  • The disease is not simply managed but is actually cured!
  • No anesthesia is required, indeed, treatment amounts to an injection followed by 3 to 7 days of boarding, very non-stressful for older cats with potential heart disease.
  • If a cat is one of the unlucky 3 to 5% for which the thyroid tumor is malignant, the initial pertechnetate scan will indicate this right away.

Disadvantages of this Method of Treatment

  • Owner and pet are separated during the quarantine.
  • Some facilities require the cat to be confined indoors or have limited contact with owners for a period of time after discharge from the facility. Children and pregnant women can have no contact with the cat for a week or two after therapy. If this is too inconvenient to work out at home, the cat may be boarded until this period has passed.
  • Facilities with capability of performing radiotherapy may not be conveniently located.
  • This is a relatively expensive therapy.
  • Special flushable cat litter is required for 1 to 2 weeks after therapy.
  • Some follow-up blood testing is generally recommended after treatment (typically 1 and 3 months after therapy).
  • There is a chance (less than 5%) that the cat will become HYPOthyroid after treatment, requiring daily oral thyroid hormone supplementation.
  • Radiotherapy may not be a good idea for a cat with poor kidney function.

If kidney function is not thoroughly investigated prior to this therapy, latent kidney failure may be unmasked irreversibly by this therapy. This can be avoided simply by screening potential candidates for kidney failure prior to recommending radiotherapy. Those who have possible kidney insufficiency should be treated with medication to bring the thyroid levels under control. If kidney function begins to show deterioration on this therapy, medication is discontinued the need for treating thyroid disease must be reevaluated. If kidney function remains stable on treatment with anti-thyroid medications, then a more permanent therapy (such as radiotherapy) can proceed.

Radiotherapy Facilities

See a list of facilitiesknown to us in the United States and Canada that can offer radiotherapy treatment for your cat.


Surgical Treatment for Feline Hyperthyroidism

The goal here is to remove the abnormal thyroid tissue, leaving the normal adjacent tissue alone.
Considering that the average hyperthyroid cat is a geriatric patient with potential for high blood pressure and heart disease, quite a bit of patient preparation is necessary to reduce anesthetic risk.

Patient Preparation

  • First, the patient’s excess thyroid level is brought into the normal range with 2 to 4 weeks of oral medication (usually methimazole - see the oral medication page). During the last 1 to 3 weeks of this period, heart medication (propranolol or other beta-blocker) is often used to compensate for the heart disease associated with hyperthyroidism, especially in cats with resting heart rates greater than 220 beats per minute. After thyroid levels have normalized, it is important to watch for an exacerbation of renal disease as it could be unmasked by the treatment of hyperthyroidism. Concurrent kidney problems complicates anesthesia and may even mean surgery is not an option.
  • Ideally a nuclear medicine scan (see radiotherapy) is done prior to surgery to determine with certainty which thyroid lobe should be removed or if both should be. If this is not feasible, the surgeon will have to make a decision based on the visual appearance of the glands during surgery.

In 30% of cases, one gland is obviously abnormal and one gland is obviously normal (or even atrophied). The decision about what to remove is easy and the risk of calcium crisis (see below) is not significant.

In 70% of cases, both glands are abnormal and must be removed. In 15% of these cats, though, the enlargement is not symmetrical and, unless a nuclear medicine scan is done prior to surgery, the surgeon may be fooled into leaving an abnormal gland behind. Alternatively, if both glands are removed, there is potential to damage the tiny but very important parathyroid glands that sit atop the thyroid glands.

Advantages of Surgical Treatment

  • Treatment is generally permanent after recovery. Unless complications arise, no further treatment is needed.
  • No special facilities are required for this treatment, unlike radiotherapy, so it is likely that someone who performs this surgery is located conveniently to any owner of a hyperthyroid cat.

Disadvantages and Potential Complications of Surgical Treatment

  • Performing surgery and general anesthesia on a geriatric patient with potential heart disease has inherent risk.
  • If abnormal thyroid tissue is left behind, hyperthyroidism is likely to recur within 6 to 24 months (note: even if normal thyroid tissue is left behind, it is possible for the normal tissue to develop abnormal changes and for the condition to begin afresh). Continued monitoring of T4, typically twice a year, is frequently recommended.
  • The monitoring before and after surgery, plus the surgery itself, is relatively expensive.
  • The laryngeal nerve is located near the thyroid gland. If it is damaged during surgery, the cat can experience a voice change. This change may be permanent.
  • The sympathetic trunk (another neurologic tissue) is located near the thyroid gland. If it is damaged during surgery, the eye on that side may develop what is called Horner's syndrome. This eye will squint and draw back into the socket, the pupil will constrict and the third eyelid will come up. This syndrome may be permanent.
  • Because this treatment represents a permanent cure, it is important that the patient’s kidney function be thoroughly evaluated prior to therapy. The effect of restoring normal thyroid function on the patient’s kidneys must be evaluated before beginning permanent resolution of the thyroid disease.
  • It takes 1 to 3 months for thyroid blood levels to stabilize following surgery. Some cats become hypo thyroid after surgery and must take thyroid supplementation tablets either temporarily or permanently.

Calcium Crisis

This is a serious risk for cats who must have both thyroid glands removed. The blood calcium level must be regulated tightly for normal muscle contraction (including heart muscle) to be possible. This regulation is controlled by the tiny parathyroid glands adjacent to the thyroid glands. If they are damaged, calcium crisis is a possible risk.

  • Signs of hypocalcemia include anxiety, appetite loss, twitching, and facial itch. These signs can progress to seizures, hence the importance of monitoring calcium levels.
  • Blood calcium should be monitored daily for 4 to 7 days following surgery. A mild reduction is considered normal but should the level drop below 7 mg/dl, the veterinarian must be prepared for seizures.
  • Calcium crisis is treated with calcium supplementation and Vitamin D but, as there are several forms of both these products, it is important that you use only products recommended by your veterinarian. DO NOT ATTEMPT TO TREAT THIS YOURSELF WITH A VITAMIN D SUPPLEMENT FROM THE DRUG OR NUTRITION STORE!
  • Supplementation may be needed for only a few days or for life depending on the damage to the parathyroids.

Medication For Hyperthyroidism (Oral)

The most common medication prescribed to treat feline hyperthyroidism is called methimazole (trade name Tapazole or Felimazole). This medication has virtually replaced the older medication propylthiouracil or PTU because methimazole is effective without as much tendency for side effects. In the United Kingdom and Australia, carbimazole is sometimes used. Because carbimazole is converted to methimazole in the body, the information here for methimazole also holds for carbimazole.

All these medications block the production of T4 and T3. Thyroid hormones that are already in the body when medication is started are still in play, so a good 2 to 4 weeks are needed before thyroid blood tests will show the effect of treatment.

It is important to understand what the potential side effects of methimazole and its relatives are, and the monitoring that is generally recommended. The thyroid nodule, which can be detected in a cat's throat, will not reduce in size with treatment and may in fact get larger.

Advantages of Using Methimazole

  • Medication is inexpensive relative to radiotherapy or surgery.
  • Control of thyroid disease is achieved only while the pet is on medication so that if there is any problem with exacerbated poor kidney function, treatment can be discontinued.
  • No hospitalization is required.
  • Side effects are relatively uncommon.
  • If an occasional dose is skipped, no harm is done.
  • If no side effects are encountered after the first 3 months of therapy, the chance of side effects occurring thereafter is substantially reduced.

Disadvantages of Using Methimazole

  • Medication must be given at least daily (usually twice daily). Some cats simply will not take oral tablets at this frequency. Methimazole is readily made into a flavored liquid or chewable for easier administration by a compounding pharmacy or it may even be possible to convert methimazole into a gel administered on the hairless inner surface of the cat’s ear (see below).
  • Approximately 15% of cats will experience some kind of side effect. The usual side effects are: lethargy, loss of appetite, and vomiting. If one of these side effects occurs, medication is discontinued until the symptoms resolve. Medication is then restarted at a lower dose and gradually increased to the former dose. These side effects do not generally recur if medication is increased gradually in this way.
  • Facial itching is a more serious side effect. This side effect also resolves with anti-itch medication and discontinuing methimazole. Cats who have this side effect can be expected to have it again if medication is restarted, so another form of treatment should be used. Facial itching occurs in less than 4% of cats on methimazole.
  • Serious liver failure results in an extremely small number (less than 2%) of cats taking methimazole. This toxicity can be expected to resolve after discontinuation of the medication but, again, alternative therapy should be considered.
  • Bone marrow changes can also result from methimazole administration. Blood tests evaluating white blood cell patterns should be periodically performed to monitor for these changes. This side effect occurs in less than 4% of cats on methimazole and necessitates a change in therapy.
  • Pre-existing kidney insufficiency can be masked in hyperthyroidism because the heart disease and high blood pressure that go with hyperthyroidism increase blood flow through the kidneys, making the kidneys more efficient; this is virtually the only positive aspect of having hyperthyroidism). Once treatment is instituted for hyperthyroidism, the kidney disease is unmasked or made worse when kidney blood flow returns to normal. Sometimes it is necessary to choose between treating the kidneys and treating the thyroid, thus monitoring kidney function and thyroid levels is particularly important during methimazole therapy. Kidney problems can be minimized by starting with a lower dose of methimazole and working up over weeks or months so as not to cause as abrupt a change in kidney blood flow. If kidney problems become significantly worse on methimazole, medication can be discontinued. Approximately 15-22% of cats treated for hyperthyroidism will show kidney disease that was not evident prior to treatment.
  • A study published in the February 15, 2006, Journal of the AVMA by Milner et al found that when cats with pre-existing renal disease are excused from the group and compared to cats treated with radiotherapy vs. those treated with methimazole, those treated with radiotherapy had a much longer median survival time (4 years vs. 2 years). This may be due to difficulties in regularly medicating cats with oral medication and ultimately leading to periods of incomplete treatment.


Periodic blood testing to examine T4 level, white blood cell patterns, kidney function, and liver enzymes should be periodically performed. Be sure to ask your veterinarian to review an appropriate schedule for your cat. Side effect potential can be reduced by beginning at a smaller dose and working up to the full therapeutic dose over the first couple of months of therapy.

Transdermal Methimazole

Many cats remain untreated for this ultimately debilitating disease because their owners cannot administer the medication. For many cats, even converting the medication into liquid does not yield a comfortable alternative. Yet another choice has become available to make methimazole a viable choice for fractious cats: a gel of methimazole administered to the hairless skin of the inner ear flap. It is important to realize that the GI tract (where pills enter the body) is designed to absorb while the skin (where transdermal medications enter) is designed to serve as a barrier. This fundamental concept has raised many questions about the efficacy of transdermal medications. A study by Sartor, Trepanier, Kroll, Rodan and Challoner published in the September/October Journal of the American College of Veterinary Internal Medicine compared T4 levels (and other parameters) in 17 cats receiving oral methimazole vs. 27 cats on transdermal methimazole. They found fewer GI side effects (vomiting, diarrhea etc.) in the cats receiving transdermal methimazole but other side effects were the same in both groups. After 2 weeks of therapy, more cats on oral methimazole had responded to medication than had the cats on transdermal medication, but by 4 weeks the transdermal cats seemed to have caught up. This study had enough cats drop out of the study by 4 weeks, however, that whether the closing of the gap was truly of statistical significance. This means that the jury is still out on the effectiveness of transdermal methimazole though it looks like there is less potential for stomach upset with this route.

Is there an Alternative to Methimazole?

It seems there are always some cats who cannot take methimazole due to side effects and cannot undergo radiotherapy due to expense. An alternative medication called ipodate (an iodine-based radiographic contrast agent) has been investigated. This medication inhibits the conversion of T4 to T3 (unlike methimazole, which inhibits production of T4). In a study of 12 hyperthyroid cats, 8 responded well to ipodate with no negative side effects. More severe cases of hyperthyroidism tend not to respond. This seems like an attractive alternative, except that nearly as soon as the research was published, the product went off the market, having been surpassed in radiology by more efficient contrast media. A similar compound called iopanoic acid appears to be similar in effect to ipodate but is difficult to obtain. It is available through compounding pharmacies, if at all.

Another approach for cats who cannot take methimazole involves a combination of potassium iodate and a heart medication called propranolol. Potassium iodate blocks the thyroid gland’s uptake of iodine. The propanolol is used to control the heart disease that is common in hyperthyroid cats, and the dose depends on the patient’s heart rate. Side effects of potassium iodate consisted of upset stomach and depression in 13 out of 20 cats and were controlled by reducing the dose. While all cats in this study seemed better with medication, less than 50% achieved normal lab values; furthermore, all the cats of this study took these medications in preparation for thyroid surgery so long-term followup was not conducted. Still, for a cat who cannot tolerate more conventional treatment, this may be a reasonable alternative.

Specifics on protocols for either of the above therapies are outlined in The Options for Treating Hyperthyroidism by Dr. D. Bruyette in the November 2004 issue of Veterinary Medicine.

It is our policy not to give dosing information over the Internet.


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