1st May 2006

What are the best tests available to check for hypothyroidism in dogs?

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Question:

What are the best tests available to check for hypothyroidism in dogs? My dog had a four-hour TRH stimulation test: Total T4 20-30 mnol/1 (20-60) Total T4 (post) 24.70 mnol/1. I was told that he had tested positive, and the vet prescribed thyroxin. My dog has been taking this for five months now, and there has been no improvement, despite an increase in dosage. He has a damaged liver caused by an unknown source and is not on any medication. Could the test be invalid because his liver is not functioning correctly? He has also been tested twice for Cushing's disease, but both tests proved negative. Any advice would be greatly appreciated.

Answer:

A definitive diagnosis of hypothyroidism can be difficult because of the many clinical abnormalities associated with thyroid hormone deficiency and the lack of readily available diagnostic tests with high sensitivity and specificity. Thyroid function tests should be performed only on dogs with clinical findings consistent with hypothyroidism. Measurement of serum total thyroxine (T4) concentration is a useful initial screening test, since most hypothyroid dogs have values below the reference range. Serum-free T4 concentration measured by equilibrium dialysis is a more sensitive and specific test of thyroid function than total T4 and is particularly useful in dogs with non-thyroidal illness or atypical clinical signs. The gold standard for diagnosing hypothyroidism remains the thyroid-stimulating hormone response test. This is the test your dog has had. It should be used to confirm hypothyroidism when other tests do not agree with the clinical impression or if atypical signs or non-thyroidal illness exists, or there has been administration of drugs known to alter thyroid function tests. Ultimately, a positive response to treatment with thyroid hormone replacement is confirmation of a correct diagnosis. The fact that your dog has liver disease may affect the results of some of the tests of thyroid function and may be an example of a non-thyroidal illness. You should discuss this possibility with your vet.