Thyroid illness in cats and dogs

Common clinical signs of hypothyroidism in dogs include weight gain, lethargy, reduced exercise tolerance and poor coat quality. However, the disease affects almost all body systems and as a result a wide variety of signs can potentially be associated with this endocrinopathy.
Diagnosis is complicated by the fact that many non-thyroidal illnesses (NTI) may present with similar features to hypothyroidism and can result in reduced thyroid output. There is no perfect lab test for the diagnosis of hypothyroidism and the available alternatives all have their own strengths and weaknesses that need to be taken into account. Often the best approach is to use multiple tests that complement one another and help to improve diagnostic accuracy.

Where hypothyroidism is a consideration, making the diagnosis requires three distinct steps:

1. Exclude non-thyroidal illness (NTI).

By excluding other disease conditions first the predictive value of the thyroid tests will be significantly increased. In practical terms this phase may include a profile including FBC and smear evaluation and where necessary screening tests for Cushings. The latter can be important as both endocrinopathies can result in alopecia, weight gain and marked reductions in T4.

2. Run a screening test for hypothyroidism.

Basal T4 is sensitive but not specific for hypothyroidism. As a result, a clearly normal T4 reliably excludes hypothyroidism in almost all cases. Basal T4 is included in the CTDS Canine Endocrine Profile. While almost all dogs with hypothyroidism have a reduced basal T4, many dogs with NTI have low T4 also. To help improve the specificity of T4 it is often coupled with TSH assay. T4 with TSH works well as a relatively low cost screening test for hypothyroidism and can be run alongside or following the endocrine profile.

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About the Author

Nick graduated from Edinburgh Veterinary School in 1980 with an
Honours degree in Pathological Sciences and in 1982 as a Bachelor
of Veterinary Medicine and Surgery. In 2003 Nick became a diplomate
of the Royal college of Pathologists in veterinary clinical pathology.