Sheldon: Understanding your dog’s thyroid gland
“The results are in for Don’s thyroid test, but I am having an endocrinologist review them.” I told Ms. H. “Thyroid testing isn’t just a positive or negative result, and, as with most endocrinology conditions, it is often confusing.”
What is your dog’s thyroid gland anyway and why would you want to understand it? Well, if your dog has an endocrine or hormone issue, most likely it is going to be the thyroid gland — so pay attention, grasshopper.
Hormones are the chemical messengers produced by your pet’s many endocrine glands. The thyroid gland, located in the neck, is probably the most influential and commonly affected gland. No diss meant to the other glands.
Hypothyroidism, the disease name for low thyroid hormones is present in about 1 in 200 dogs (0.5%) but finding accurate data here is sketchy. It can occur in any dog, but some breeds are predisposed, meaning we see more of it in them. These are golden retrievers, dobermans, dachshunds, shelties, Irish setters and cocker spaniels.
Thyroid hormones’ most important roles are in metabolism and skin homeostasis (a fancy word for a stable physiological environment). Most of the common symptoms of hypothyroidism are obesity, lethargy, poor/dull skin and coat, and chronic ear and skin conditions. Less commonly seen are moodiness, seizures or neurologic issues and even gastrointestinal symptoms. The range of symptoms is so wide that I have pretty much seen almost any condition caused by low thyroid hormones.
As we first discussed, testing can be a little confusing. The most common test is a simple total T4 (TT4) measurement which can be done in your veterinarian’s office. The problem is that a low or even low normal result does not necessarily mean hypothyroidism as so many factors and medications play a role in the TT4 level. A normal level of TT4 pretty much correlates with a normal thyroid but a low or low-normal level does not necessarily mean your dog has hypothyroidism.
TT4 is included on many routine panels but it does add a little to the cost. If I strongly suspect hypothyroidism, I move straight away to the Michigan State University Endocrinology labs’ full thyroid panel. Other labs run this panel but MSU is the best in my opinion and their lab is easy to work with. This panel is 2-3 times more expensive than just a TT4 but still affordable.
What we are looking for in the full panel are a few hormones, but free T4 (FT4) is the most important. FT4 is the active form in the tissue. TT4 is bound by protein and not available to the cell until it is released, then it is called FT4. This is only partially accurate though, as the active form inside the cell is T3, which comes from FT4.Confused yet?
The MSU panel also includes TT3, FT3, TSH, and thyroid antibodies. TSH is an interesting discussion too. Called thyroid-stimulating hormone, it is the message from the pituitary gland, located in the brain, to make more thyroid hormone. So, if TSH is high, it means the pituitary gland thinks there is not enough T4. The problem here is in about 25% of the cases, TSH levels really do not correlate well with thyroid disease. How is your confusion level now?
And what about measuring T3? I just said it is the most important part of the thyroid equation. Too bad, so sad: T3 measurements really mean little, it is affected by too many variables, so the measurement is deemed inaccurate. Still confused?
The good news in all of this is that it is quite easy to treat hypothyroidism. Simply replace the hormone with a drug called thyroxine, check T4 levels in a few weeks to make sure you have the right dose and voila, problem solved. You could also check FT4 but that means sending blood to the lab and it is much more expensive. Well, in my experience, 75% of problems gone. Yeah, I know, confusing.
Some veterinarians, myself among them, will treat a dog with a 60-90 day trail of thyroxine when the tests are in the gray area but who shows strong clinical sign and symptoms of hypothyroidism. Most doctors are taught to treat the patient and symptom; not the piece of lab paper. In this case, excess thyroid hormone for 90 days is rarely if ever harmful. Endocrinologists, by the way, are OK with this trial.
As you have read, hypothyroidism in dogs can be a tad confusing. The good news is that it is rarely if ever a life-threatening condition.