Veterinarians often say that they see patients in clusters. One week might be "diabetes" week; the next is all about inflammatory bowel disease. Sometimes these clusters are real, like in cases of an outbreak of infectious disease, but more often than not they are probably just a chance occurrence. Whatever the reason, this month has been all about the thyroid gland for me.

I’ve already talked at length about my two hyperthyroid cats; let’s ignore them for the moment. The opposite problem, hypothyroidism, is much more common in dogs, but it is not always a straightforward diagnosis. Let’s look at the reasons why.

The thyroid gland manufactures a hormone that essentially sets a dog’s metabolic rate. When the thyroid gland does not secrete enough of this hormone, usually because it has been destroyed by an abnormal immune reaction, a dog’s metabolism sloowwws waaaay doowwwn. Typical symptoms of hypothyroidism include:

  • weight gain
  • lethargy
  • hair loss
  • recurrent infections
  • heat-seeking behaviors
  • and in severe cases, seizures or other neurologic problems, a thickening of the skin producing a "tragic" facial expression, and sometimes tendon or ligament injuries.

If your dog has some of these symptoms, blood work has revealed low thyroid hormone levels, and other conditions causing similar clinical signs have been ruled out, a tentative diagnosis of hypothyroidism is appropriate. I say "tentative" because the last stage of diagnosis should be response to treatment. If your dog’s symptoms improve with thyroid hormone replacement therapy after recheck blood work has confirmed that therapeutic levels have been reached, you now can be confident that your dog truly was hypothyroid.

Problems arise when a dog’s symptoms and lab work don’t match up well. Why? Because dogs that are sick with diseases completely unrelated to the thyroid gland often develop low thyroid hormone levels. The condition is called euthyroid sick syndrome, and it does not require thyroid hormone replacement therapy. What is really needed is an accurate diagnosis and treatment aimed at the underlying problem, but this is sometimes easier said than done!

Also, treatment with some types of drugs (e.g., prednisone, phenobarbital, and sulfa antimicrobials) can result in low thyroid hormone readings and some breeds (e.g., greyhounds) naturally have a relatively low level of thyroid hormone in their blood stream. All this can cause dogs to be misdiagnosed with hypothyroidism when something else entirely (or nothing at all) is actually wrong with them.

The screening test for hypothyroidism is called a TT4 for Total T4. T4 is the form that thyroid hormone takes when it is travelling through the blood stream, and it is easy and inexpensive to measure. If your dog has a low TT4 but his symptoms don’t correlate well with hypothyroidism (especially if he is losing weight — always question the diagnosis if your dog is losing weight), it’s time for more diagnostic testing. The best confirmatory tests are a Free T4 by Equilibrium Dialysis, Endogenous Canine Thyrotropin (cTSH) Concentration, and/or Thyrotropin (TSH) Response Test. The results of one or more of these will usually differentiate true hypothyroidism from euthyroid sick syndrome and the other causes of falsely low TT4s.

If you have any reason to question your dog’s diagnosis of hypothyroidism, particularly if it was based primarily on a low TT4 level, ask your veterinarian to run a Free T4 by Equilibrium Dialysis, cTSH, or TSH response test.

Dr. Jennifer Coates

Image: Ollie In Blanket by Chilean / via Flickr