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Why Are My Dog 's T3 Levels Abnormal ?

Why Are My Cat 's T3 Levels Abnormal ?

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

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

To see how tests are often grouped, go here

Ron Hines DVM PhD

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Triiodothyronine Levels In Your Pet's Blood


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

Your veterinarian will run the T3 test on your dog or cat when he/she is interested in how well its thyroid gland is working. 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 by far, has 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 dependent on the amount of T3 that is eventually available.

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

Your pet’s T3 levels can be low in many non-thyroid health problems. 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 one of the most common errors in veterinary medicine.

T3 (along with TSH level) can be helpful in deciding if sight hounds (eg greyhounds, etc.) are truly hypothyroid as they often run low T4 levels even when they are really normal. In those breeds, low T3 is a better indication of hypothyroidism than low T4.

Reasons Why Your Pet’s T3 Levels 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 antibodies because they will not respond as expected to ordinary thyroid medications.

Reasons Why Your Pet’s T3 Levels Could Be Low :

When pets are Hypothyroid (Primary hypothyroidism) their serum or plasma concentrations of T3 will be low or undetectable.

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

Low T3 syndrome is a common situation that accompanies a wide variety of diseases of dogs and cats that has nothing to do with the pet’s thyroid gland. Veterinarians probably over diagnose hypothyroidism. 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. (All our sick dolphins at SeaWorld had low T4 levels.) Once the underlying problem is dealt with, the T3 levels of all animals return to normal.

What If My Pet’s Thyroid Results are Borderline or Normal But My Pet Shows Symptoms of Hypothyroidism ?

Many veterinarians believe that “sub-clinical” or low-normal hypothyroidism exists in dogs. When several unexplainable symptoms, commonly associated with hyperthyroidism, are present in your pet but its thyroid function tests are still within the laboratories “normal” range", some veterinarians are comfortable giving a low test dose of thyroid medication for a few weeks to see if the pet improves. Those pets need close monitoring.

Complementary Tests :
CBC/WBC and blood chemistry values, thyroid panel, thyroglobin auto antibody test, TRH stimulation test, T3 suppression test for cats

.................... DxMe