Hypothyroidism in Dogs
The recommended blood test for screening purposes is the total T4. This test
is indicated for dogs who have findings suggestive of hypothyroidism on
physical examination. A normal T4 is fairly conclusive evidence that the dog
does not have hypothyroidism. However, a low-normal or below-normal level does
not mean the dog is hypothyroid, because concentrations below normal are common
for many reasons other than hypothyroidism.
To avoid overdiagnosing and overtreating the disease, it is important to
confirm the significance of a low T4 using a more accurate thyroid function
test, such as the FT4 by equilibrium dialysis. Other blood tests are also
available for diagnosing hypothyroidism. One is an assay for thyroglobulin
autoantibodies; these autoantibodies are present in about 50 percent of dogs
with autoimmune thyroiditis. This test must be sent to a special laboratory for
Treatment: Hypothyroidism is permanent, but can be effectively treated with
daily or twice-daily thyroid hormone replacement using synthetic L-thyroxine
(L-T4). The initial dose is based on the dog’s weight. This should be adjusted
for individual circumstances. Monitoring is done by physical examination and
measuring the total T4. This must be done frequently, particularly early in the
course of treatment. Hair loss and other signs of hypothyroidism usually
reverse with treatment. A few dogs will need T3 supplementation as well.
The Orthopedic Foundation for Animals (OFA) maintains a hypothyroid registry
to identify dogs as normal for breeding (see appendix D). A certificate and
breed registry number is issued to all dogs found to be normal at 12 months of
age, based on the results of FT4, cTSH, and thyroid autoantibody screening by
an OFA-approved laboratory. Screening and registration is of value for dogs at
risk of inherited hypothyroidism. If the test is positive, these dogs should
not be used for breeding.
Michigan State University also has a thyroid registry and keeps statistics
on thyroid function in dog breeds.