An Ultimate Guide to Hyperthyroidism in Cats

Cats are cute creatures. Several health-related conditions affect the overall well-being of cats. Just like humans, the thyroid gland plays an important role in keeping a cat sound and healthy. Thyroid gland issues can become serious for your beloved furry friend, if not addressed properly. Many cat owners ask questions about the “Hyperthyroidism in cats” such as,

What are the important causes of hyperthyroidism in cats?

What are the key signs and symptoms of hyperthyroidism in cats?

How can hyperthyroidism be managed in cats?

In this article, we will discuss the important causes and signs of “Hyperthyroidism in cats under the light of scientific literature and general field experience.


Hyperthyroidism is also called “Thyrotoxicosis”. It is a thyroid gland complication that arises due to over-secretion of thyroid hormones (over-functioning of the thyroid gland), particularly T3 and T4. Keep in mind that, this is the most common glandular issue in cats that usually occurs in middle-aged and senior cats.


Normally, the thyroid hormones are involved in regulating the many-body processes in cats, particularly metabolic activities. Increased secretions of thyroid hormones accelerate the metabolic processes which result in abrupt weight loss in cats with increased appetite.


There are several signs and symptoms of hyperthyroidism in cats. Here we have summarized some important ones.

  1. Increase in appetite(hyperorexia)
  2. Abrupt weight loss (Important key sign)
  3. Increased thirst (Polydipsia)
  4. Dramatically increased heart rate (Tachycardia)
  5. Irritability, restlessness, and hyperactivity
  6. Dull/poor hair coat
  7. Difficulty in breathing
  8. Aggression in behavior
  9. Generalized weakness (in exceptional cases)

Important note: Increase in appetite and weight loss is two very important clinical signs which are seen in many cases. However, in exceptional cases, lack of appetite, dullness, and lethargy have also been observed in many cats.

Last but not least, vomiting, panting and increased urination have also been seen in some cases.


1) Hyperactivity of thyroid gland

2) Thyroid gland tumors (Thyroid adenocarcinoma)

3) Nodular hyperplasia (enlargement of the thyroid gland)

Note: In many cases, enlargement of the thyroid gland is non-cancerous


This condition can occur in any gender (male and female cats). Mostly, this condition occurs in old cats (12-13 years old). It has also been documented that hyperthyroidism occurs in those cats who already have underlying complications. A very rare occurrence has been reported in young cats.


The clinical picture of hyperthyroidism can be correlated with diabetes, kidney failure, inflammatory bowel disease etc.

Blood tests:

Complete blood tests and urinalysis also help in determining the increased levels of thyroxin in the cat’s blood. However, this must be kept in mind that, many cats show mild hyperthyroidism (they have normal levels of thyroxin in the blood). The thyroid hormone level can vary from time to time. So, it is better to test it regularly along with other tests (Liver function tests and kidney function tests).

Physical examination of the Thyroid gland:

Another way to diagnose hyperthyroidism in cats is to check the thyroid gland. A clearly enlarged thyroid gland can be sensed in the neck of the cat.

Nuclear medicine testing:

For a more accurate diagnosis, nuclear testing can help. In this method, a small dose of the radioactive compound is injected into the cat. This radioactive compound travels through the blood and reaches the abnormal thyroid gland. If the thyroid gland is hyperactive, it will accumulate a greater amount of radioactive compound and vice versa. the enlarged thyroid gland scanned by the radioactive compound can be clearly seen with the help of a camera. A Technetium scan is the best prototype of nuclear testing.


Several treatments are available to reverse this condition in cats. For example, anti-thyroid medications

(carbimazole and methimazole ). This is better to discuss it with a registered veterinary practitioner. He/she can suggest you the best treatment plan according to the current clinical presentation of your cat.

Also there is CATalyst which is a Grade-A supplement formula the helps supports your cat’s joint function, immune system, digestive health as well as hyperthyroidism.

Iodine therapy:

This is another awesome treatment protocol that gives success rates. In short, radioactive iodine is used in this treatment strategy. A calculated dose of iodine is given under the skin of an affected cat. Iodine travels to the abnormal, hyperactive thyroid gland and accumulates there. This is how; it helps in destroying the abnormal areas of the thyroid gland. Note, there are not many side effects of this treatment but it requires proper care and well-designed centers.

Surgical intervention:

This is also a good option. In this method, the abnormal or affected area of the thyroid gland is removed surgically (thyroidectomy). This method also requires expert practitioner’s care.

Other treatments include provision dietary supplements to cats to limit the over-production of thyroid hormones. Keep in mind; never use any medication on your cat without taking the proper prescription from a registered veterinarian.


 How longer a cat with hyperthyroidism lives?

It depends upon the management and medical attention that is given to her. Normally, a cat with proper treatment can survive up to three to five years (average).

Does hyperthyroidism cause other complications in cats?

Yes, if not properly managed, hyperthyroidism can lead to kidney and heart failure in cats.

Why cats with hyperthyroidism lose weight?

Due to increase metabolic rate (excessive burning of energy), the cat becomes emaciated.


Broussard, J.D., Peterson, M.E. and Fox, P.R., 1995. Changes in clinical and laboratory findings in cats with hyperthyroidism from 1983 to 1993. JOURNAL-AMERICAN VETERINARY MEDICAL ASSOCIATION206, pp.302-302.

Graves, T.K., Olivier, N.B., Nachreiner, R.F., Kruger, J.M., Walshaw, R. and Stickle, R.L., 1994. Changes in renal function associated with treatment of hyperthyroidism in cats. American journal of veterinary research55(12), pp.1745-1749.

Martin, K.M., Rossing, M.A., Ryland, L.M., DiGiacomo, R.F. and Freitag, W.A., 2000. Evaluation of dietary and environmental risk factors for hyperthyroidism in cats. Journal of the American Veterinary Medical Association217(6), pp.853-856.

Shiel RE, Mooney CT. Testing for hyperthyroidism in cats. Veterinary Clinics of North America: Small Animal Practice. 2007 Jul 1;37(4):671-91.