Hyperthyroidism in Cats
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Article Written by Dr. Daniel A. Degner, Board-certified Veterinary Surgeon (DACVS)

Key Points

This common condition affects cats in the geriatric years. It is caused by an overactive thyroid gland, which becomes enlarged and produces excessive thyroid hormone

Treatment involves medicines, radioactive iodine or surgery

Prognosis is generally very good following treatment

Anatomy and physiology

  • The thyroid is a paired set of glands located in the mid to upper neck region, adjacent to the windpipe. The thyroid glands produce two hormones: thyroxin and calcitonin. Thyroxin is a hormone which controls metabolism. Too much of this chemical messenger and the metabolic rate increases dramatically, which ultimately will lead to diseased body organ systems. Too little this hormone results in low metabolism and also a diseased state of the body. Another hormone calcitonin stimulates the bones to draw calcium from the blood stream.
  • Another set of glands which produce hormones are the parathyroids. There are two parathyroid glands closely associated with each thyroid gland, making a total of four parathyroids. Each thyroid gland has an internal parathyroid gland, which is imbedded deep in the thyroid tissue. The external parathyroid gland is located usually on the upper end of the thyroid gland, but sometimes it is also embedded in the thyroid or is at the lower end of the thyroid. The parathyroid glands produce parathyroid hormone, which regulates the calcium level in the blood. If there is a lack of this hormone, the calcium in the blood stream will decrease to levels which are life threatening.
  • A variety of nerves and vessels are intimately associated with the thyroid glands. The recurrent laryngeal nerves are located on each side of the windpipe and cause the voice box to open and close with respiration and swallowing. The vagosympathetic trunk lies with a main artery (carotid) that runs in the neck to the head and brain. The vagosympathetic trunk has a variety of functions, one of which is to dilate the pupil.



  • This common condition affects cats in the geriatric years. It is caused by an overactive thyroid gland, which becomes enlarged and produces excessive thyroid hormone. The result is excessive thyroid hormone production and stimulated body systems. When the heart is stimulated by excessive hormone levels, it beats faster and the muscle becomes thickened. In some cases congestive heart failure results. The predominant clinical signs of this include shortness of breath, exercise intolerance, coughing, and paralysis to the hind limbs (if a blood clot dislodges from the heart). High blood pressure can result, which can cause blindness and seizures.
  • The kidneys, when affected by excessive thyroid hormone, work overtime. Therefore, hyperthyroidism can mask preexisting kidney failure. As a result, if hyperthyroidism is treated such a cat could go into a state of kidney failure. If there is any concern about your cat having pre-existing kidney disease, definitive treatment such as surgery or radioactive iodine therapy should be considered with great caution.
  • The most common clinical signs of hyperthyroidism are increased appetite with concurrent weight loss. These cats usually beg for food all of the time. One must remember that these clinical signs are also seen with other conditions. Another clinical sign is agitated behavior such as pacing and meowing. An uncommon form of hyperthyroidism results in apathy and lethargy.



  • The diagnosis of hyperthyroidism is based on measuring the level of thyroid hormone in the blood. Usually cats with an elevated thyroid hormone level have some of the aforementioned clinical signs. Sometimes cats that truly have an overactive thyroid and have clinical signs which support this, do not have a markedly elevated thyroid hormone level on a blood test. If the thyroid hormone level is on the high end of normal and your veterinarian can feel an enlarged thyroid, hyperthyroidism is present. Sometimes other diseases in the body can cause the thyroid level to not be as high as expected, thus masking a diagnosis of hyperthyroidism. In addition some cats will have undulating thyroid levels, thus when the blood is taken from the pet it may be at its low level. Taking another blood sample for testing, is therefore advisable. If the thyroid level is still not above the normal high range, then a thyroid scan with nuclear imaging can be done. Another test that can be done is a thyroid suppression test, but the results can be sometimes equivocal.
  • The cat that has confirmed hyperthyroidism should have complete testing of the blood with a complete blood cell count and chemistry profile. The urine should also be tested. If a heart murmur or gallop rhythm is detected an ultrasound of the heart and chest x-rays should be taken.



  • Medial therapy involves administration of medication called Tapazole on a daily basis. This medication will reduce the circulating thyroid hormone released by the abnormal thyroid nodule. Problems with this type of treatment include reactions (vomiting, loss of appetite, low platelet and blood cell counts, liver failure) and the fact that the cat needs to be pilled daily (inconvenience). Blood work should be closely monitored when your pet receives this medication to make sure that the internal organs continue to function normally and the blood counts remain stable.
  • Another medical treatment is to feed the affected cat a severely iodine deficient diet called Hill's y/d. This seems to be very effective treatment and cats seem to find the food to be very pallitable. No other treatment is needed in these patients if the thyroid level normalizes.
  • Surgery involves usually removal of both left and right thyroids. About 70% of cats that are hyperthyroid have abnormal thyroid tissue in both glands, even if only one appears to have a tumor. There is a 98% chance that the thyroid tumor is benign or noncancerous. The remaining 2% have malignant tumors. The parathyroid gland must be carefully preserved during the surgery, in order to prevent low calcium levels of the blood. An alternative to removing both left and right thyroid gland is to remove the largest of the two, then six weeks later remove the remaining thyroid gland. By staging the surgery, the parathyroid gland which is pealed off of the thyroid gland will recover its function within a period of 2 to 3 weeks if it has been insulted. In the event that the blood supply to the parathyroid gland is damaged while it is dissected from the thyroid gland, the parathyroid can be implanted into a muscle and it will regain function over a period of 3 weeks. In my opinion staging the thyroid gland removal results in a less worrisome aftercare and the cat can go home usually on the day of the procedure. It is also my opinion, because surgery to remove thyroid glands is a quick procedure with minimal morbidity, it should be strongly considered over other treatments.  In the photo below a large thyroid tumor is being removed.
  • Radioactive iodine131 can be injected intravenously in order to treat hyperthyroidism. This medication is taken up by the active thyroid nodule and the radioactivity will destroy the abnormal tissue and leave the normal thyroid glandular tissue intact. One of the disadvantages of this technique is that the pet must be kept in strict isolation and not sent home for a minimum of 1 to 3 weeks. During this time of isolation the pet should have minimal contact with humans due to the radioactivity. In spite of this, it is a very safe and effective technique.



Complications of Surgery

  • If both thyroid glands are removed at the same time there is a significant risk that the parathyroids could be injured and become temporarily nonfunctional. The calcium levels will drop and result initially in loss of appetite and dilation of the pupils. As this condition worsens twitching of the ears occurs. Finally twitching of the muscles of the entire body and seizures occur, which can result in death if not treated. The low blood calcium is treated initially by giving intravenous calcium and then administering an oral vitamin D type of medication called calcitriol. By staging removal of the thyroid glands, this complication can be eliminated.
  • Breathing difficulties may occur if the recurrent laryngeal nerves are damaged during the procedure. Another cause of breathing difficulty is congestive heart failure associated with hyperthyroidism. I have never seen this complication.
  • Hypothyroidism or abnormally low thyroid hormone level may occur. This is unusual as most cats have other smaller ectopic thyroid glands which take over the function.
  • Kidney failure may result after surgery if hyperthyroidism was masking the kidney failure. A trial of Tapazole can be administered in suspect kidney failure cats. If the kidney function does not deteriorate with this medication, then radioactive iodine therapy or surgery should be safe.
  • Recurrent hyperthyroidism can take place if the other thyroid is not removed or if any of the cells from the removed thyroid nodule are left behind.


Complications of Radioactive Iodine

  • Cats that are frail or need close care due to other pre-existing conditions may succumb due to the lack of monitoring and treatment that would have otherwise been given.
  • Hypothyroidism is a bit more common with this type of therapy than surgery, especially if the I131 dose is slightly too high for the cat. This results in a lethargic pet that gains weight easily.
  • Recurrent hyperthyroidism
  • In general this treatment is very safe and effective

Frequently Asked Questions After Surgery - General Information

When should my cat have the first bowel movement after surgery?

  • Many cats will not have a bowel movement for the first 4 to 5 days after surgery
  • Reasons that a cat will not have regular bowel movements after surgery include:
    • The cat has been fasted prior to surgery
    • Cats do not eat well during the hospital stay
    • They frequently do not eat well when the go home
    • They are fed highly digestible food that produces little stool
    • Pain medication that contain narcotics (such as fentanyl patches, tramadol, morphine) can be constipating
  • If a pet does not have a bowel movement on the 5th day of being home a stool softener such as metamucil can be fed
    • Dose of metamucil 1/4 tsp per mixed in with each meal (canned cat food)

My cat had surgery and will not eat.  What can be done?

  • Offer smelly foods that contain fish such as tuna or smelly cat foods
  • Try Gerber strained meats for babies such as the chicken, beef, turkey or veal
  • Hand feeding;  place a small amount of food in the mouth so that they get the flavor
  • Warm the food slightly in a microwave as the food will be more aromatic; remember to stir the food before feeding and test the temperature with your finger; it should  be only luke-warm.
  • Some cats will only eat dry food, try kibble if your cat normally has been fed that food
  • Petting and stroking your cat frequently will help to stimulate appetite
  • Remember that most pets will not eat the first day or two after they get home from surgery
  • Appetite stimulants such as cyproheptadine may be helpful
  • If your cat refuses to eat anything for 7 days a stomach tube should be placed to provide nutrition so that a serious liver problem (hepatic lipidosis) does not develop

My cat is vomiting now that he/she is at home.  What can be done?

  • The first thing for you to discern is whether your pet is vomiting or regurgitating.  Both will result in fluid or food being brought up.  Vomiting always will have heaving or retching of the abdomen prior to expulsion of the vomitus.  Regurgitation is not associated with heaving and the cat usually just opens the mouth and fluid or food will be expelled.  Usually the regurgitant will be clear or brown colored fluid. 
  • Next is to identify the cause of the vomiting or regurgitation.
  • Causes and treatment of vomiting after surgery
    • When some pets return home after a stay in the hospital they may drink excessive amounts of water at one time and then vomit; if this appears to be happening the water should be limited to frequent smaller amounts.
    • Medications such as antibiotics are a common cause of vomiting after surgery.  In order to see which medication is causing the problem the administration of each drug should be separated 2 hours apart.  Usually the pet will vomit or appear nauseated (drooling and sick look) within 1 hour of administration of the medication that they are sensitive to.  The antibiotic in some cases may be changed to a different one, or may be discontinued. 
    • Stomach upset from anesthesia is a potential cause of vomiting and will pass within a couple of days. 
    • Unusual cause of vomiting after surgery is internal organ failure.  Blood testing will confirm this problem. For this reason vomiting should not be ignored if it persists for more than 24 hours.
    • If your pet had surgery of the bowels or stomach, vomiting is always a concern, as it may indicate that infection of the abdominal cavity, called peritonitis, is present.  Do not ignore this sign.
    • Symptomatic treatment of vomiting involves with holding food for 12 to 24 hours, then introducing small amounts of bland food such as rice and lean cooked hamburger, if your pet does not vomit after that then.  In order to decrease the acidity of the stomach Pepcid AC 0.5 mg/kg given by mouth twice daily for 5 days can sooth an upset stomach.  Metoclopramide is a good anti-vomiting medication for cats.  You should always consult a veterinary healthcare professional before administering medication.
  • Causes and treatment of regurgitation after surgery
    • The most common cause of regurgitation is reflux of acid from the stomach into the esophagus while your pet is under anesthesia.  Acidic fluid from the stomach can cause a chemical burn of the esophagus and result in a bad case of heart burn, which is called esophagitis.  This results in poor motility of the esophagus so water and food will accumulate in this structure.  In most cases esphagitis is self-eliminating and will resolve within two or three days. 
    • Regurgitation also can be caused by a neuromuscular degeneration of the esophagus and this problem will persist.  It is not associated with surgery, rather other underlying diseases.
    • If the esophagitis is severe the esophagus may develop one or more strictures.  A stricture is a narrowing or stenosis of the esophagus, does not allow passage of food down the esophagus, thus the pet has persistent regurgitation.  This problem should be brought to the attention of your doctor within the first two weeks so that it can be treated by ballooning the stricture (minimally invasive procedure as it is done with the aide of an endoscope).  If an esophageal stricture is chronic surgery is needed.
    • Symptomatic treatment of regurgitation caused by esophagitis includes feeding bland food, and administering a coating agent such as sucralfate.  You should consult a veterinary health care professional if the regurgitation continues for more than a couple of days.

How do I know that my cat is in pain following surgery?

  • Pain is more difficult to assess in cats versus dogs as signs can be more subtle and they usually do not vocalize
  • Signs of pain in a cat include the following:
    • biting if you get near the surgical site
    • growling or deep cry
    • not wanting to eat
    • hiding and not wanting to be near owner (remember that this could also be caused by the cat just being upset about leaving home and coming back)

What can be done for pain at home for my cat?

  • Pain medication such as buprenorphine or a Duragesic (fentanyl) patch
  • Tylenol will kill a cat as they lack abundant glutathione enzyme in the liver
  • Anti-inflammatories can be used, but the dose is much less than dogs and they should be given only for a few days

Is it okay for my cat to lick or scratch the incision?

  • If a cat licks the incision it will actually delay the healing process because they usually lick too much and traumatize the area.
  • Cats have a barbed tongue, therefore a lot of damage can be done in a short period of time
  • Licking can remove stitches and cause the incision to open
  • Licking can become a severe habit that is difficult to break
  • Licking can cause infection as the mouth has many bacteria
  • Cats will frequently lick the incision when the owner is not watching such as at night time; if the skin looks red or excoriated the most common cause is from licking.
  • To stop your pet from licking/scrtaching the following can be tried:
    • Elizabethan collar can be placed on the neck; this will not help stop your pet from scratching at the region
    • Cervical collar (bite not collar) is a less awkward device and can be effective at stopping a pet from licking the surgical site
    • If the incision is over the chest an infant tee shirt can be put on your pet and the waist of the shirt fastened in place with an ace bandage or duct tape.
    • If the incision is over the paw or lower limb a bandage or sock could be put on and kept up with tape.
    • Bitter apple can be applied around the incision; many dogs will continue to lick  after application of this topical
    • Bitter Apple and Liquid HeetTM (obtain this from a drugstore...it is used for sore muscles) mixed in a 2:1 ratio can be applied around the skin incision
    • Antipsychotic medication in some cases is needed

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  • Four years of advanced training in surgery beyond the Doctor of Veterinary Medicine Degree

  • Experience in the development of new surgical treatments

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  • Assurance that a veterinarian is a surgical specialist

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