A variety of thyroid function tests is offered by Antech Diagnostics either alone or as part of a comprehensive total health screening panel. Currently, our recommended approach for new patients is to include a complete thyroid antibody profile as part of the comprehensive health screening panel.

Antech's thyroid hormone assays use radioisotopic methods, except for the thyroglobulin autoantibody (TgAA), which is measured by ELISA. All reported reference ranges refer to those of adult animals.

Measuring serum T4 alone is considered to be imperfect for diagnosis of thyroid disease, because it can: overdiagnose hypothyroidism; underdiagnose hyperthyroidism; 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, some nonsteroidal anti-inflammatory agents).

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 or two-step solid phase or liquid-phase analog RIAs; enzyme-linked immunosorbent assay (ELISA); and chemiluminescence. The advantages and disadvantages of current free T4 assays have been vigorously debated. Methods used routinely at Antech Diagnostics include a one-step RIA method and the dialysed [ED] RIA method.

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 spuriously raises the T3 level.

Like the free T4, free T3 represents the biologically active fraction of total T3. Levels are usually normal in hypothyroid dogs without concurrent NTI, may be elevated slightly in dogs with increased tissue metabolic demands, are low in dogs with NTI, and are typically spuriously high or very high in dogs with T3 autoantibodies.