Hypothyroidism is the most common endocrine disease of the dog. Confusion remains, however, over which diagnostic tests are the most specific and sensitive for identifying thyroid dysfunction especially in its early stages. Accurate diagnosis of the early compensatory stages of canine hypothyroidism and autoimmune thyroiditis affords important clinical consequences for early therapeutic intervention and case management. Additionally, the heritable nature of autoimmune thyroiditis in dogs has significant genetic implications for those breeds.

Important diagnostic considerations include: patient history and clinical signs, family and breed history, age, stage of estrous cycle for intact females, and current physical findings and treatments. Because of the variety of individual tests available today, with their inherent strengths and weaknesses, it is often difficult to make a definitive diagnosis. Dr. David Panciera (1997) succinctly captured this situation by stating "a healthy dose of skepticism should accompany interpretation of any thyroid function test, with evaluation of the history and physical examination findings being paramount to an accurate diagnosis". For the above reasons, use of more comprehensive thyroid panels is strongly recommended.

Measuring serum T4 alone is considered by most experts to be unreliable for diagnosis of thyroid disease, because it can: overdiagnose hypothyroidism; underdiagnose hyperthyroidism; may fail to detect early stages of the compensatory disease; and cannot identify the presence of thyroiditis. This test is greatly influenced (lowered) by the presence of nonthyroidal illness (NTI) and specific drug therapy (e.g. corticosteroids, anticonvulsants, potentiated sulfonamides, nonsteroidal anti-inflammatory agents).

The serum free T4 represents the small (< 0.1%) biologically active fraction of the total T4, and is therefore less likely to be influenced by NTI.

As a single test, accurate measurement of free T4 has been shown to have the highest sensitivity, specificity, and accuracy for diagnosing canine hypothyroidism.

The techniques used in veterinary medicine for assaying free T4 include: direct radioimmunoassay (RIA) determination after equilibrium dialysis (ED), considered by many to be the "gold standard"; one-step solid phase or liquid-phase analog RIA; enzyme-linked immunosorbent assay (ELISA); and chemiluminescence. The advantages and disadvantages of current free T4 assays have been vigorously debated. Currently, methods used routinely at Antech Diagnostics include the ED and solid-phase analog RIA, both of which have been validated repeatedly, and correlate well in most cases.

Measuring serum T3 alone is also not recommended, although it can be useful as part of a thyroid profile or health screening panel. For example, if levels of total and free T4, and total T3 are all low, the patient most likely has a NTI rather than hypothyroidism. If total T3 levels are high or very high in a dog not receiving thyroid supplement, the patient most likely has a circulating T3 autoantibody (the most common type), which has spuriously raised the T3 level.

Like the free T4, free T3 represents the biologically active fraction of total T3. Levels may be elevated slightly in euthyroid dogs with increased tissue metabolic demands, and are typically spuriously high or very high in dogs with T3 autoantibodies.

In primary hypothyroidism, as serum free T4 levels fall, pituitary output of thyroid stimulating hormone (TSH) rises in a regulatory, compensatory response. In human medicine, highly sensitive and accurate endogenous TSH assays are available which make diagnostic testing straightforward, as virtually all hypothyroid patients have elevated TSH levels. However, in veterinary medicine the new canine-specific TSH assays are not as sensitive or specific as the human ones. When measured as part of a complete thyroid panel or along with free T4, the cTSH assay offers another analyte to more completely assess canine thyroid function. Recent experience with this test indicates about 20-30% discordancy between expected and actual results in normal dogs as well as in confirmed cases of hypothyroidism or NTI. Two recent studies showed that the specificity of diagnosing hypothyroidism increased when the cTSH assay was combined with either free T4 or T4 assays, but the sensitivity and accuracy of diagnosis were lower when cTSH was measured alone. Thus, the cTSH assay by itself is not recommended for diagnosing canine hypothyroidism, and spuriously low or high cTSH levels can be seen in some hypothyroid or euthyroid dogs, respectively.