VET TIPS: Hyperthyroidism Common In Older Cats

By  //  February 4, 2014

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Veterinary Medicine

(VIDEO by PetCareTV)

Although thyroid abnormalities are not uncommon in humans, many people are surprised to learn that dogs and cats can have thyroid disease too.

BREVARD COUNTY • MELBOURNE, FLORIDA — Although thyroid abnormalities are not uncommon in humans, many people are surprised to learn that dogs and cats can have thyroid disease too. 

The thyroid gland is located in the neck and plays a very important role in regulating the body’s rate of metabolism. Hyperthyroidism is a disorder characterized by the overproduction of thyroid hormone and a subsequent increase in the metabolic rate. 

This is a fairly common disease of older cats.  Although the thyroid gland enlarges, it is usually a nonmalignant change (benign).  Less than 2% of hyperthyroid cases involve a malignancy.

Contributing Factors

Advancing age is the main factor that increases a cat’s risk for hyperthyroidism.  Environmental and dietary risk factors have been investigated and may play a role in predisposing cats to hyperthyroidism, though the specific mechanisms are not known.

The Siamese breed has a ten-fold lower risk of developing hyperthyroidism.

No individual breed is known to be at increased risk, although the Siamese appears to have a 10-fold lower risk of developing hyperthyroidism than other breeds.

Clinical Signs

The typical cat with hyperthyroidism is middle-aged or older; on the average, affected cats are about 12 years of age.  The most consistent finding with this disorder is a loss of weight secondary to the increased rate of metabolism.

The cat tries to compensate for this with an increased appetite.  In fact, some of these cats have a ravenous appetite and will eat anything in sight!  Despite the increased intake of food, most cats gradually lose weight.  The weight loss may be so gradual that some owners will not even realize it has occurred.

There may be periodic soft stool or diarrhea, and the hair coat may be unkempt.   In some cats, a loss of appetite develops as the disease progresses.

Two secondary complications of this disease can be significant.  These include hypertension (high blood pressure) and a heart disease called thyrotoxic cardiomyopathy.  Hypertension develops as a consequence of the increased pumping pressure of the heart.

In some cats, blood pressure can become so high that retinal hemorrhage or detachment will occur and result in sudden blindness.  The heart problems develop because the heart must enlarge and thicken to meet the increased metabolic demands.  Both of these problems are reversible with appropriate treatment of the disease.


Palpation of a large benign thyroid nodule (adenoma) in a hyperthyroid cat.

In most instances, diagnosis of this disease is relatively straightforward.  Sometimes an increase in size of one or both thyroid lobes can be palpated (felt) during the physical exam.  A blood test measures the level of one of the thyroid hormones, called thyroxine (or T4).

Usually, the T4 level is so high that there is no question as to the diagnosis.  Occasionally, a cat suspected of having hyperthyroidism will have T4 levels within the upper range of normal cats.  When this occurs, a second test, to measure the unbound metabolically active, or free T4 may be performed.

Treatment Options

Because less than 2% of these cats have cancerous growths of the thyroid gland, treatment is usually very successful.  There are four choices for treatment, any one of which could be the best choice in certain situations.

Many factors must come into consideration when choosing the best therapy for an individual cat.

  1. Radioactive iodine.  The most effective way to destroy all of the abnormal tissue is with radioactive iodine therapy.  It causes no damage to normal thyroid tissue or to the nearby parathyroid gland.  The treatment consists of one injection, followed by 3-4 days of hospitalization to allow the radioactivity to attenuate.This small amount of radiation does not cause hair loss, vomiting or any other side effects.  Ninety-six percent of the patients treated with radioactive iodine require no retreatment and no thyroid supplementation.
  2. Surgery.  Surgical removal of the affected thyroid lobe(s) is another effective treatment.  Because hyperthyroid cats are usually over 8 years of age, there is a degree of risk involved.  However, if the cat is otherwise healthy, the risk need not be considered significant.  If the disease involves both lobes of the thyroid gland, two surgeries may be required, depending on the surgeon’s choice of procedures.  In many cats, only one thyroid lobe is abnormal, so only one surgery is needed.  If both glands are removed, there may be complications from damage to the parathyroid glands, which are located on and within the thyroid glands.  This could lead to low blood calcium levels, resulting in seizures and death if not treated.
  3. Oral medication.  Administration of an oral drug, methimazole, can control the effects of the overactive thyroid gland.  Some cats have reactions to the drug, but that number is fairly small (less than 20%).  However, the side effects may begin as late as six months after the beginning of treatment and can include vomiting, lethargy, anorexia, fever, and anemia.  Methimazole does not destroy the abnormal thyroid tissue, but rather ties up the excess thyroid hormone.  Therefore, the drug must be given for the remainder of the cat’s life.  Periodic blood tests must be done to keep the dosage regulated.
  4. Prescription diet y/d.  This diet became available in October 2011.  It is very low in iodine compared to other diets.  Since iodine is required to make thyroid hormone, it starves the tumor by not providing it with the raw materials to make thyroid hormone.  Early studies show that this diet is a safe and effective method of controlling hyperthyroidism, but regular testing is still necessary, and it is critical that the cat eat NOTHING except the prescribed diet.  Even one treat a day could contain enough iodine to allow the tumor to produce excessive thyroid hormone.


Many owners of cats with hyperthyroidism are hesitant to have radiation therapy or surgery because of their cat’s advanced age.  But remember, old age is not a disease.  The outcome following both surgery and radiation therapy is usually very positive, and most cats have a very good chance of returning to an excellent state of health for many years.


Dr. Jeff Godwin graduated in the Charter Class at the University of Florida College of Veterinary Medicine in 1980. He has been practicing in Melbourne since 1981. Dr. Godwin enjoys small animal medicine and orthopedic surgery, and is the Hospital Director for his practice, Animal Medical Clinic of Melbourne. 

Dr. Jeff Godwin
Dr. Jeff Godwin

He is a past president of the Brevard County Veterinary Medical Association, the Florida Veterinary Medical Association, and the Melbourne Rotary Club and is the Past Chairman of the Melbourne-Palm Bay Chamber of Commerce. He and his wife Jo have an empty nest except for their Yorkshire Terrier, “Tebow.” In his spare time he enjoys hiking, camping, and backpacking.

Dr. Godwin was selected as the Florida Veterinarian of the Year in 2002 and has served as the Treasurer of the Veterinary Orthopedic Society since 2007.

You can reach Dr. Godwin at The Animal Medical Clinic, 4020 South Babcock Street, Melbourne, Florida, 321-727-2421.