Hyperthyroidism in Humans and Animals

Posted on March 4th, 2015

Hyperthyroidism in humans and animals comes from a thyroid gland that is over producing its thyroid hormones T3 (triiodothyronine) and T4 (thyroxin).

When the production of T3 and T4 are deficient, the opposite condition occurs, which is referred to as hypothyroidism.

An inflammation of the thyroid gland in humans can lead to the production of elevated T3 and T4 also.

This condition is referred to as thyrotoxicosis.

Hyperthyroidism can also occur due to over supplementation of thyroid supplements.

Hyperthyroidism occurs in humans and cats mainly and is rarely seen in dogs.


  1. Nervousness = H & C
  2. Irritability = H & C
  3. Increased sweating = H
  4. Increased heat rate = H & C
  5. Heart murmur = C
  6. Cardiac arrhythmias = H
  7. Anxiety = H & C
  8. Insomnia = H & C
  9. Thinning hair = H
  10. Hair loss = H
  11. Muscle weakness = H & C
  12. Increased bowel movements = H
  13. Significant weight loss = H & C
  14. Increased appetite = H & C
  15. Increased thirst = H & C
  16. Vomiting = C
  17. Diarrhea = C
  18. Panting = C
  19. High blood pressure = H & C
  20. Neurological signs like tremors, chorea and myopathy = H


  1. Toxic thyroid adenoma = H
  2. Grave’s Disease (autoimmunity) = H
  3. Toxic multi-nodular goiter = H
  4. Thyroiditis (inflammation of the thyroid gland) = H
  5. Over supplementation of thyroid hormones = H & C
  6. Excess secretion of thyroid stimulating hormone (TSH) = H - Note: usually from a pituitary tumor. Recent findings have indicated that elevated TSH may also play a role in cats with hyperthyroidism, but the reason for this is still unknown.
  7. Benign tumors = C
  8. Goiter (enlarged thyroid gland) = H & C - This is the most common cause of hyperthyroidism in cats. The incidence of goiter in cats has been reported to be as high as 70%.
  9. Polybrominated Diphenyl Ether (PBDE’s) = C
  10. Bisphenol A (BPA). This comes from the plastic that lines canned cat food containers. It also occurs in many different types of plastics, including plastic baby bottles. (NOTE: The environmental exposure of cats to this contaminant is highly significant.)

Exposure to many household products like flame retardants, furniture polish and electrical products may produce a state of hyperthyroidism in cats.

Certain foods that are fed to cats may contain toxic levels of PBCE, which comes from fish, liver and giblets.

For more information on these environmental toxins and the damage they cause, please read, Adrenal Toxicity and Hormonal Immune Destabilization in Animals by Dr. A. J. Plechner, Journal of Toxicology, Vol. 24, Issue 1, January 8, 2004.


  • The clinical symptoms in humans and the clinical signs in cats
  • The main blood tests which usually confirm hyperthyroidism in humans are TSH, T3 and T4.
  • The main blood test done in cats to confirm a diagnosis of hyperthyroidism is usually only a T4 and a thyroid equilibrium test.

Note: Normally hyperthyroidism needs to be diagnosed, not only based upon the clinical signs, but also based upon elevated T3 and T4 hormone levels.

Too often a diagnosis of hyperthyroidism is made in a cat, based solely upon an elevated T4 level.

It is highly important to include a T3 test also, and both an elevated T3 and T4 are necessary to make absolutely sure the cat is hyperthyroid.

If only T4 is tested, without knowing what the T3 is, if the T4 is elevated and the T3 is decreased or normal, the cat has a transference problem and is not hyperthyroid. This transference problem stems from a cortisol imbalance and if a cortisol replacement is given, the T4 will transfer into T3.

If this is the case, and only the T4 has been tested and is elevated, there is a good chance that the therapy, used to treat hyperthyroidism, may be detrimental and the cat’s condition will not improve.

If this might be a concern, a simple endocrine immune test can be done, which includes cortisol, T3, T4 and total estrogen.

For more information on this blood test, please go click here.


  • Surgery
  • Radioiodine therapy
  • Anti-thyroid drugs

Your health care professional will help you decide which treatment protocol might work best.

These are only my thoughts and I hope they will help you and your cat live a longer, healthier life.


Dr. AL Plechner