Hyper vs Hypothyroidism
January 2024

One of these conditions we see in clinic more often than the other but what is the difference?

Hyperthyroidism – Is a condition commonly seen in older cats and very rarely in dogs. Affected patients have an overactive thyroid gland which results in increased production of the thyroxine hormone (T4)[1] and triiodothyronine (T3)[2]. These hormones act in regulating the animal’s metabolic rate (the speed in which the machine runs). High production of these hormones leads to an increase in metabolic rate. Patients with this condition show symptoms such as increased hunger, increased thirst (fuel for the speed machine) and weight loss. A fast metabolic state also increases the workload of the cardiovascular system, so symptoms such as an increased heart rate or an enlarged heart may also be seen[3]. Thankfully there are treatment options available to “slow the machine” which include radioactive iodine treatment, anti thyroid medications (Vidalta or Methimazole), surgery to remove the glands or changing to a prescription diet (y/d)[4].

Hypothyroidism – This condition is commonly seen in middle aged mid to large breed dogs and rarely in cats. These patients have a decreased production of the thyroid hormones leading to a slower or sluggish metabolism (like starting an old car on a frosty morning). This leads to symptoms such as lethargy, poor coat condition, hair loss, trouble maintaining body warmth and weight gain without an increase in diet. In non-spayed females with hypothyroidism there is a decrease in litter survival rates due to the necessity of thyroid hormones for growth and development of the pups. There is also an increased risk of both male and females to become infertile due to this hormone deficiency[5].Treatment involves supplementation of thyroxine and regular appointments to check T4 levels and improvement of presenting symptoms[6]

So Hyperthyroidism is a speed machine vs Hypothyroidism which may need an oil change, full service and supplemented petrol!

[1]BSAVA Textbook of Veterinary Nursing 6th edition Ed. B Cooper, E Mullineaux andL Turner. Chapter 18 by R Gear pg. 568

[2] The thyroid gland in animals by MPeterson Ed. 2022 https://www.msdvetmanual.com/endocrine-system/the-thyroid-gland/the-thyroid-gland-in-animals?query=thyroxine

[3] Disorders of the thyroid gland incats by M Peterson and J Kretchevsky Ed. 2022 https://www.msdvetmanual.com/cat-owners/hormonal-disorders-of-cats/disorders-of-the-thyroid-gland-in-cats

[4] https://www.hillspet.com.au/cat-food/pd-yd-feline-dry

[5] Disorders of the thyroid gland indogs by M Peterson and J Kretchevsky Ed. 2022 https://www.msdvetmanual.com/dog-owners/hormonal-disorders-of-dogs/disorders-of-the-thyroid-gland-in-dogs

[6]BSAVA Textbook of Veterinary Nursing 6th edition Ed. B Cooper, E Mullineaux andL Turner. Chapter 18 by R Gear pg. 569