Over Active Thyroid In Cats

Hyperthyroidism, over active thyroid, in the most common hormone imbalance in cats. Cats have two thyroid glands, one on either side of the trachea in the neck and play an instrumental role in the body’s metabolic rate. Hyperthyroidism is characterized by overproduction of the thyroid hormone, which then results in increased in the body’s metabolic rate. The disease can affect cats of any age however it is predominantly noted in older cats. As time progresses the thyroid gland enlarges and can often be felt when your cats next stretched back. In fewer than 2% of cases the enlargement of the thyroid gland involves a malignant cancer therefore in most cases the overall enlargement is not of major concern. However the results of the increase in circulating thyroid hormone are of concern, particular to the heart and the kidneys.

The primary cause for hyperthyroidism is not known but there are some studies to believe that increases in iodine in diet may play a role in its prevalence. There is not a particular breed predisposed to hyperthyroidism, although Siamese cats seem to be some of the most common patients affected.

An image giving example of where the thyroid gland is in a cat.
An image giving example of where the thyroid gland is in a cat.

The symptoms of hyperthyroidism include weight loss despite a healthy appetite and often increased appetite. Cats affected by the disease are often restless, and may have increased cranky or aggressive behaviors. In the later stages of the disease some cats even stop eating altogether. Secondary complications to the disease include high blood pressure, which has detrimental effects on the heart and the kidneys. The heart condition is known as thyrotoxic cardiomyopathy. The increased blood pressures cause increased work on the heart ending in over use of the muscles and thickening of the chambers. The long term increased blood pressure causes similar destruction to the kidneys, resulting in kidney failure.

Hyperthyroidism can be diagnosed by complete blood testing that many veterinarians may be able to perform right in the office, or sending samples to an outside laboratory. It is strongly recommended to get all baseline blood work first in addition to a T4 level and urinalysis. Occasionally, a cat that is suspected of having hyperthyroidism has a TT4 level within the upper range of normal. When this occurs, a second test, usually either a Free T4 by Equilibrium Dialysis (FT4 by ED) or a T3 Suppression Test is performed.

There are four treatment options for hyperthyroid patients:

  1. Surgery: This is where a portion of the thyroid, or one of the pairs is removed. Prior to having the procedure done a cat is placed on an oral medication called Methimazole to determine if there are any secondary diseases (liver or kidney disease).
  2. Oral Medication: Long-term management can also be achieved by administering an oral medication once to twice daily for the rest of the patient’s life. This is often the preferred choice for many pet parents given the initial costs are a lot less, however it may be a more expensive treatment option in the long run. 
    Felimazole is an approved medication to treat over active thyroid in cats.
    Felimazole is an approved medication to treat over active thyroid in cats.
  3. Radioactive iodine: This treatment is a very effective plan in which radioactive material is injected that subsequently destroys the thyroid gland.   The treatment does however require a one to two week hospitalization stay at a center certified in the treatment and handling or patients. Some of the negatives would include over destruction of the thyroid gland, require the patient to go on supplementation for the rest of their life.
  4. Dietary: In recent years a prescription food has been introduced to manage hyperthyroidism. This food is made by Hill’s Science Diet and called y/d diet. The food is not considered a drug, but has the precise amount of iodine in the food as to not permit over active thyroid function. In order for the food to work it must be the only food fed, thus no treats or special snacks.

Hyperthyroidism can be easily managed and monitored under the watchful eye of your family veterinarian. Early detection is key, and therefore it is strongly encouraged that cats over the age of 6 years old have annual blood work performed. Please talk to your veterinarian if you have concerns that your feline companion has thyroid disease.