Why Are My Dog And Cat’s T3 Thyroid Hormone Levels Low Or High?

Why Are My Dog And Cat’s T3 Thyroid Hormone Levels Low Or High?

Ron Hines DVM PhD

To see what normal blood and urine values are, go here

For an explanation of causes of most abnormal blood and urine tests, go here

To see how tests are grouped, go  here

Meds that might interfere with thyroid gland test results here

One low thyroid test doesn’t mean you pet is hypothyroid? Read about that here

T3 = Triiodothyronine – The  Level In Your Dog Or Cat’s Blood

Hypothyroidism is considerably more common in dogs than cats. Hyperthyroidism is considerably more common in cats than dogs.

Your veterinarian might run the T3 test on your cat or dog when he/she is interested in how well its thyroid gland is functioning. Your pet’s thyroid gland produces two forms of thyroid hormone, T3 and T4. The thyroid gland produces most of it as T4, which is later converted to T3 in other locations in the body. Of the two, T3 has by far the more powerful effect on your pet’s body. Your cat or dog ’s metabolic rate, its heart and respiratory rate and its body temperature are all very dependent on the amount of T3 that is eventually made available. All supplemental thyroid hormone given to thyroid-deficient pets, is T4 (levothyroxine).

A complete thyroid panel analysis consists of five or six assays to judge the condition of your pet’s thyroid gland. Those test check the amount of T3, T4, free T4 (by equilibrium dialysis), TSH and thyroglobin auto antibody that is present in the blood. Knowing all four values helps your veterinarian decide the underlying cause of your pet’s thyroid problems. It also helps your vet monitor the effectiveness of treatment. The tests can also be used by certifying organizations (such as the OFA) to pre-screen potential breeder animals for any genetic tendency toward thyroid gland problems.

Your pet’s T3 levels can also be reported as low in many non-thyroid health issues. When it is found to be low, some of those other five tests need to be run to be certain the dog or cat is truly hypothyroid. Placing non-hypothyroid dogs on thyroid medication is not an uncommon error.

T3 (along with TSH level) can be helpful in deciding if sight hounds (eg greyhounds, etc.) are truly hypothyroid. These swift breeds often have T-4 levels that would be considered low in other breeds. But in sight hounds they are actually normal. In sight hounds low T3 levels are a better indication of true hypothyroidism than low T4 levels.

Reasons Why Your Cat Or Dog’s T3 Level Could Be High:

T3 levels are usually elevated (or on the high end of “normal”) in cats with hyperthyroidism. However, in some hyperthyroid cats, it is primarily T4 that is elevated.

Dogs that have the typical symptoms of hypothyroidism but who have normal or unexpectedly high levels of T3 (and/or T4) may be producing antibodies (auto antibodies) that destroy their own thyroid hormones. Those dogs need to be tested for the presence of those anti-thyroid antibodies because they will not respond as expected to ordinary thyroid medications.

Reasons Why Your Dog Or Even Your Cat’s T3 Levels Might Be Low:

When pets are hypothyroid (primary hypothyroidism) their serum or plasma concentrations of T3 will often be low or even undetectable.

The same situation can occur in cats or dogs that have had their thyroid glands removed or irradiated as a treatment for hyperthyroidism or thyroid tumors or when they received too high a dose of methimazole (Tapazol®) to suppress their thyroid gland’s hormone (T4) production.

As I mentioned earlier, low T3 syndrome is a common situation that accompanies a wide variety of diseases of dogs and cats that have nothing to do with the pet’s thyroid gland abilities. That is why veterinarians probably tend to over diagnose rather than under diagnose hypothyroidism in dogs. In human beings, more than 70% of patients in intensive care units are said to have low T3 levels and around 50% have low T4 levels as well. Neither veterinarians nor physicians know why this occurs. (Most of our sick dolphins and whales at SeaWorld had low T4 levels that were not associated with thyroid gland disease.) Once the underlying health problem is dealt with successfully, the T3 and T4 levels of all animals should return to normal.

What If My Dog’s Thyroid Results are Borderline Low or Normal But My Pet Is Showing Symptoms of Hypothyroidism?

Many veterinarians believe that “sub-clinical” or low-normal hypothyroidism exists in dogs (hypothyroidism of any kind is quite rare in cats). When several unexplainable symptoms commonly associated with hypothyroidism are present in your pet, but its thyroid function tests are still within the laboratory’s “normal” range”and all other diagnostic tests are normal there is another option. Some veterinarians are comfortable giving a low test dose of thyroid medication (levothyroxine)  for a few weeks to see if your pet improves. Those pets need close monitoring.

Complementary Tests:

CBC/ WBC and blood chemistry values , thyroid gland test panel (=T3, T4, TSH) , thyroglobin auto-antibody test, TRH stimulation test , T3 suppression test for cats


You are on the Vetspace animal health website