Euthyroid Sick Syndrome in Dogs: When Is Hypothyroidism not Hypothyroidism?

Hypothyroidism is one of the most common endocrine (hormonal) diseases I see in dogs.

Euthyroid Sick Syndrome in Dogs: When Is Hypothyroidism not Hypothyroidism?

It occurs most frequently in middle-aged (4-10 years old) medium to large-breed dogs. Breeds that are commonly affected include Doberman Pinschers, Golden Retrievers, Irish Setters, Miniature Schnauzers, Dachshunds, Cocker Spaniels, and Airedale Terriers.

Hypothyroidism occurs when the thyroid gland secretes less thyroid hormone than normal.

It usually is caused either by

  • inflammation that destroys the normal thyroid tissue, or
  • the replacement of normal thyroid tissue with fatty tissue.

Because thyroid hormone has an effect on many parts of the body and the disease is slowly-progressive, the signs may be variable and non-specific at first.

Owners may notice

  • weight gain
  • lethargy
  • intolerance of exercise
  • heat-seeking behavior due to a lowered metabolism
  • skin and hair coat changes (e.g., dull hair coat, hair loss, increased skin pigmentation)
  • skin infections may be evident also
  • other possible signs include neurologic and/or reproductive abnormalities

However, these symptoms are not specific for hypothyroidism and can be caused by a variety of diseases.

Diagnosing hypothyroidism is not straight- forward.

The supporting signs and symptoms must be backed up by laboratory tests. Routine testing (such as a complete blood count and a biochemical analysis) may provide only supportive evidence of thyroid disease or may reveal another disease that is mimicking thyroid disease.

More specific thyroid function tests (measurements of total T4, free T4, and Thyroid Stimulating Hormone or TSH) can show that the thyroid gland is normal. However, they do not conclusively prove that a dog’s illness is due to thyroid gland dysfunction.

Many non-thyroidal diseases and medications can cause a syndrome known as Euthyroid Sick Syndrome

This syndrome results in reduced total T4 concentrations and mimics a hypothyroid condition

  • thyroid levels are low
  • the dog may have some of the symptoms commonly seen with hypothyroidism, and
  • clinical improvement may be seen following administration of thyroid medication (which is the treatment for hypothyroidism)

However, the thyroid gland is actually functioning normally.

Any severe systemic disease can cause low thyroid concentrations including neurologic diseases, heart failure, renal disease, and cancer. Some medications that can cause Euthyroid Sick Syndrome include

  • phenobarbital
  • zonisamide
  • sulfa antibiotics
  • clomipramine
  • aspirin
  • corticosteroids (including topical formulations)

To complicate matters further, some breeds of dogs normally have low serum total T4 and free T4 concentrations including

  • Greyhounds
  • Scottish Deerhounds
  • Other sight hounds
  • Alaskan sled dogs (particularly during intense training or racing)

The most specific test that is currently available for differentiating hypothyroidism from the Euthyroid Sick Syndrome is the measurement of free T4 by equilibrium dialysis.

It has the greatest diagnostic sensitivity and specificity – meaning that when thyroid disease is present, the free T4 is most likely low and when the free T4 is low, thyroid disease is likely to blame.

Ultrasound of the thyroid gland by an experienced radiologist and scintigraphy (a thyroid scan) can also provide useful information, but due to the cost and limited availability, these tests are not commonly used.

Veterinarians must be careful when evaluating a dog that looks like he has hypothyroidism.

An accurate diagnosis must be reached before proper treatment can start and the patient starts down the road to recovery.

13 Comments
    • Hypothyroidism is very common. It is, however, also very often misdiagnosed. As I observed with our dogs as well, the thyroid hormone levels drop with ANY disease process going on in the body.

  1. Very informative post. My eldest Husky began having age on-set seizures at 12 and it was due to hypothyroidism and hypoglycemia. My husky with epilepsy I had on Phenobarbital and also had hypothyroidism and always had to get liver checked. So I am familiar with that, but I had not heard of Euthyroid Sick Syndrome until now. Good to know.

    • It happens; I’ve seen it happen with my own dogs (two out of three). Every time they were sick in any way. thyroid hormone went down. With hypothyroidism, it is important not to miss the diagnosis but also not to jump to conclusions.

  2. I knew that dogs could have hypothyroidism, but this is the first I’ve read about Euthyroid Sick Syndrome. If any of my dogs are diagnosed with hypothyroidism, I’ll have to revisit this post.

    • It is important not to miss the correct diagnosis. When I suspected Cookie might have had hypothyroidism, I worked directly with Dr. Dodds to have good faith to have it done right.

  3. That’s really interesting. I had never heard of it before. It’s good to know what breeds commonly are more prone to it.

  4. Marjorie Dawson

    I only thought cats had hyperthyroid stuff!

    Thanks to you I know that dogs do too, and that the issue is a complex one. Reading this as a dog owner I would have a handle on what the vet might say so thank you.

    • Need to make the distinction between hyper and hypo. One is overly active, the other is insufficient. Cats typically get hyperthyroidism and dogs typically get the opposite.

  5. Very interesting! I’ve known about hypothyroidism in dogs for awhile but Euthyroid Sick Syndrome isn’t something that was on my radar before. Thank you for the information!

    • It’s a fancy word for sure. But it means that a sick dog will have low thyroid levels. I noted that on bloodwork when my dogs were sick. Not that I tested for it while my dog wasn’t well but often we went with a “lab special” which included T4 as part of the package.

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