Does My Cat Really Have Hyperthyroidism?

Posted on May 5th, 2016

What is Feline Hyperthyroidism?

This is a disease of the thyroid gland found mainly in felines. It does occur in canines but to a much lesser degree.

Probably close to 98 % of the feline patients that have hyperthyroidism, will have a functional tumor in their thyroid gland that may or may not be palpable. It is a rare that the thyroid tumor is malignant.


  • Increased appetite
  • Increased weight loss
  • Increased heart rate
  • Poor coat condition
  • Panting
  • Heat intolerance
  • Possible cardiomyopathy


  1. A BLOOD SERUM analysis in 98 % of the felines may indicate one or more of the following enzymes, to be elevated:




  1. ENDOCRINE SERUM analysis:
  • Thyroxin (T4) hormone. This seems to be the main endocrine diagnostic test that veterinarians use, to determine if a feline has hyperthyroidism.

T4 is a storage hormone only, and will never determine is a cat has hyperthyroidism, without also measuring the active thyroid hormone, T3

  • Triiodothyronine (T3) Hormone suppression test. This is a test that is usually not performed. It involves giving the patient a T3 hormone supplement and in a normal feline, the T4 hormone levels will decrease. When this T3 hormone supplement is given to a feline with hyperthyroidism, the T4 hormone level still remains elevated.


  1. Surgical removal of the thyroid gland
  2. Radioactive iodine therapy
  3. Medical treatment

Carbimazole is used in the United Kingdom

Methamizole is used in the United States.


Even if you’re feline has an elevated T4 hormone level and may be showing some similar signs of felines with actual hyperthyroidism, NEVER BEGIN ANY HYPERTHYROID TREATMENT, UNTIL THE T3 HORMONE LEVEL HAS BEEN DETERMINED!

The T3 hormone is the active thyroid hormone that is often not even measured. Did you know that a T4 hormone can be elevated, due the fact that the feline has a cortisol imbalance, originating from the middle layer adrenal cortex?

Did you realize that T4 hormone is merely storage; an inactive form of thyroid hormone?

It takes normal amounts of cortisol to transfer T4 hormone into T3 hormone and when there is a cortisol imbalance, the T4 hormone will be elevated because it is not transferred into a T3 hormone, which may remain normal or deficient.

This is NOT feline hyperthyroidism!

Providing any feline with only an elevated T4 hormone with hyperthyroid therapy could be disastrous.

Felines can also have elevated amounts of T3 and T4 hormones without showing signs of hyperthyroidism.

How does this happen?

When an endocrine immune (hormone antibody) imbalance exists, the following may happen:

  • A cortisol imbalance will lead to the production of elevated adrenal estrogen.
  • The elevated adrenal estrogen will bind the receptor sites for T3 and T4 hormones.
  • When this occurs, the metabolism of the liver and kidney is decreased.
  • This in turn allows for the normal production of thyroid hormones to continue, but because of the bound thyroid receptor sites, both the T3 and T4 hormones will increase in amounts because of the decreased metabolism of the liver. The decreased metabolism will not allow for a normal 24 hour breakdown of the T3 and T4 hormones. At the same time, because of the decreased metabolism in the kidneys, normal 24 hour excretion of the T3 and T4 hormones will not occur.

Once this imbalance has been identified and corrected, the T3 and T4 hormone levels will become normal once again.

Treating a patient for hyperthyroidism without identifying this imbalance, may prove to be disastrous.

In any event, at the very least, if your cat is diagnosed with hyperthyroidism with only a T4 hormone elevation, NEVER ALLOW ANY HYPERTHYROID THERAPY TO BEGIN WITHOUT FIRST DOING A T3 HORMONE LEVEL.

These are only my thoughts and I hope they help keep your feline healthier, happier and living a longer life.


Dr. Al Plechner