Feline Hyperthyroidism

What is hyperthyroidism?

Feline hyperthyroidism is a disorder resulting from excessive thyroid hormone. The disease occurs in middle-age to older cats without sex or breed predilection. Though functional benign enlargement (adenoma) is most common (98%), thyroid carcinoma (cancer) is another cause (2%). About 70% of cats have both lobes of the thyroid gland affected.

  • Symptoms: May include weight loss, poor hair coat, rapid heart rate, voracious appetite or thirst, anxiety or nervousness, diarrhea or vomiting, and vocalizing.
  • Diagnosis: Lump or mass in the neck detected during a physical exam; elevated levels of thyroid hormone in the blood; isotope imaging that depicts hyper-function as a change in the size, shape, and location of the thyroid glands.
  • Treatment: The four options include anti-thyroid medication, surgery, diet, and radioiodine therapy.
  • Prognosis: Generally good. Cats with severe disease involving many organ systems may not survive. However, most cats, even teenagers, respond well to treatment.




Cornell veterinarians are fully qualified to treat cats diagnosed with feline hyperthyroidism. We can suggest the best treatment for your cat and help manage the problem. We have experience with medical, surgical, dietary, and radioiodine treatment, complications, and long term management. We can assist your veterinarian with follow up care. All treatment options carry the potential risk of hypothyroidism.

Medical Treatment:

Anti-thyroid drugs inhibit the production of thyroxine by blocking reactions that produce the hormone. These drugs are usually effective but may be needed lifelong. As with any drug, there can be adverse reactions. These include loss of appetite, vomiting, depression, bleeding, liver problems, facial swelling and itching. Pills are given two times a day. Periodic exams are needed to check hormone levels and adjust the dosage. Long term therapy (years) is discouraged because the thyroid tumor continues to grow and eventually drugs may not work.

Surgical Treatment:

Removal of the thyroid gland is a surgical procedure with a good success rate. General anesthesia is used with attendant risk and older cats may have heart, kidney or other problems that cause complications. Loss of the parathyroid gland can cause problems with calcium metabolism.

Dietary Treatment:

An iodine-restricted diet (Hill's Prescription Diet Y/D) is available commercially. Consumption of this diet by a hyperthyroid cat decreases production of thyroxine by the thyroid gland by limiting the amount of iodine necessary for thyroid hormone production. Cats must eat onlythis diet lifelong with no supplementation with other foods, treats, or hunting.

Radioiodine Treatment:

GUIDELINES FOR PRE-RADIOIODINE TREATMENT: Consult your referring veterinarian about taking your cat off of Methimazole for 7-10 days and Hills YD 14 days prior to your appointment with us. 
The availability of radioiodine for cats is limited to hospitals with radioisotope permits. We have treated cats with radioiodine at Cornell for more than 25 years. Radioiodine is safe and effective with cure rates approximately 95 - 98% with one treatment. Cats can receive a second treatment, if necessary. Radioiodine treatment avoids surgery, anesthesia, and anti-thyroid drugs. A single injection is given subcutaneously (i.e. under the skin, like a vaccine) and the radioactive iodine is quickly absorbed into the bloodstream. The iodine is taken into the thyroid gland and incorporated into thyroxine. The majority of cats have normal hormone levels within a week or two of treatment. Hospitalization is required under the radioisotope permit issued by New York State to Cornell. You can expect that your cat will remain at the Hospital for 3 to 5 days after injection (about a week total). Additional minor precautions after discharge are needed but they are not difficult and will be thoroughly explained to you. 


The Cornell University Hospital for Animals is the teaching hospital for the College of Veterinary Medicine at Cornell. We train veterinary students, interns (recent DVM graduates), and residents (DVM graduates with a year or more of training) as part of our mission. A board-certified expert, experienced faculty member oversees, supervises, and has final responsibility for the diagnosis and treatment of our animal patients. We are nationally and internationally recognized as a premier college of veterinary medicine and teaching facility.

We know that your time is valuable but we ask your patience with the teaching process as students and new doctors learn through experience. You will be greeted by a third or fourth-year veterinary student and will meet both interns and residents during your visit. Faculty may also introduce themselves. Our examination will review past history, physical and laboratory results; we will conduct additional physical and laboratory examinations to confirm the initial diagnosis and make sure that no other problems exist that would complicate treatment. All tests and procedures and the related costs will be explained; you can accept or decline recommendations during these discussions. We strive to be thorough and recommend those procedures that are essential to your cat and avoid unnecessary testing.

If radioiodine is the treatment of choice, we will explain the procedure and keep you informed during hospitalization (3 – 5 days).

GUIDELINES FOR PRE-RADIOIODINE TREATMENT: Consult your referring veterinarian about taking your cat off of Methimazole for 7-10 days and Hills YD 14 days prior to your appointment with us.


Cats treated with radioiodine (I-131) have a low level of radioactivity following discharge from the hospital. This level is sufficiently low that complete isolation of the cat from people is not required. Follow these guidelines for 3 weeks after your cat comes home to eliminate or further reduce human exposure to radiation that is emitted by the cat and by radioiodine that is excreted by the cat in urine and saliva.

---Do not allow your cat to sit on or sleep with anyone after coming home. Spend as little time as possible close to your cat. Casual contact is fine, but prolonged lap sitting and sleeping in your bed should be avoided. You can relax these guidelines for yourself after the second week, but not for young children or women who are pregnant or might become pregnant.

---As an extra safety precaution for the very young and developing fetuses, children under 10-years-old and pregnant woman should avoid contact with your cat, food dishes, toys and litter for the full 3 weeks.

---Keep your cat separate from any other cats in the household in order to reduce the risk of radiation exposure. Please ensure that your other cats do not use the same litter box, as radioiodine is excreted in the urine. Please ensure that your other cats are fed from separate food bowls and water dishes as radioiodine is also excreted in the saliva.

---Wash your hands with soap and water after touching your cat or anything the cat has been in contact (e.g., food dishes, toys, bedding, and especially the litter box). When cats groom themselves saliva contaminates their hair coat, which can then contaminate dishes and other objects.

---If possible, use flushable litter for the next 3 weeks. Otherwise use scoopable litter and collect all waste in a sturdy container lined with two plastic bags (one placed inside the other). Store waste in a well-ventilated space away from your primary living areas, and hold for an additional 2 weeks so natural decay will reduce radioactivity to background levels. Then the litter may be disposed with the normal trash. Landfills do not allow the disposal of low-level radioactive waste and are equipped with sensitive radiation detectors. You may be charged over $1000 if radioactivity is detected in your cat's litter at the landfill.

---Please use extra care when cleaning the litter box to avoid getting soiled litter on your hands. Use disposable plastic gloves and litter box liners to help prevent contamination. Wash with soap and water after cleaning the litter box. All used disposable gloves should be stored with the waste—treat them as if they are contaminated.

---Please keep your cat indoors if the cat uses garden areas as a litter box. Cats may go outside for supervised exercise.

---Do not allow your cat to eat from your plate or walk on counter tops where food is prepared. If the cat does or you are not sure, then put on disposable gloves and thoroughly wash the area with soap and water before preparing food.

---Items that your cat routinely contacts (e.g., bedding, toys) should be thoroughly washed with soap and water prior to handling them without disposable gloves.

---If your cat salivates on hard surfaces, then these areas should be cleaned often with soap and water, or a spray cleaner (e.g., Formula409). For surfaces that are difficult to clean (e.g., couches and upholstered chairs) cover these surfaces with a towel or blanket and replace as necessary. These towels and blankets should be washed separately from your clothing using standard laundry detergents.

---If your cat vomits a hair ball, put on disposable plastic gloves and dispose of the material with the waste litter or flush down the toilet. Next, clean the area with soap and water or spray cleaners.

---If your cat needs emergency care within the first 3 weeks of coming home, then advise the veterinary staff that your cat has recently undergone radioiodine therapy.

---After a month (typically after your first recheck appointment with your veterinarian) you can resume all normal routines and assume that all hazards associated with radioiodine treatment are gone.



Q: My cat is on Tapazole, can she still be given radioiodine?
A: Cats should be off anti-thyroid drugs for a week before receiving I-131. This helps with the incorporation of radioiodine into thyroid hormone. If you or your veterinarian are concerned about withdrawing anti-thyroid drugs for a week because of the severity of your cat's hyperthyroidism, please contact us by calling 607-253-3060.

Q: My cat is 17, can she stand the hospitalization?
A: Yes, while disconcerting, cats quickly adjust in the hospital. We have successfully treated cats as old as 21-years-of-age.

Q: Can I visit?
A: No, this is not permitted, but you may leave blankets, toys, or special diets (no fish) for your cat.

Q: Are some cats NOT able to receive radioiodine?
Q: Yes, some cats have too many problems that require intensive nursing and handling to allow radioiodine therapy. Treated cats are housed in a separate ward to minimize exposure to radioiodine.

Q: What happens if my cat becomes critically ill while hospitalized for radioiodine therapy?
A: Full care is given with arrangements made to minimize radiation exposure to our personnel. Needed treatment is never denied.