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When a dog suspected to have hypothyroidism has a low total T4 concentration, fT4 and/or TSH concentrations must be evaluated to help confirm or refute the diagnosis (TABLE 1). If the TSH concentration is high, hypothyroidism can be diagnosed.
If T 4 concentration is at the low end of the reference interval, and hypothyroidism is strongly suspected clinically, measurement of fT 4 or TSH concentration can help to characterize the dog's thyroid function.
Total T4 (TT4) Canine Evaluation of total T4 (TT4) is best used to rule out a diagnosis of hypothyroidism or monitor dogs on levothyroxine supplementation. TT4 concentrations below the lower reference limit are suspicious (but not confirmatory) for hypothyroidism. Evaluation of FT4 and TSH is recommended, if hypothyroidism is clinically suspected.
Dogs with T4 results in the low normal range may be hypothyroid. In dogs with low or low normal T4 results and with consistent clinical signs, consider one or more of the following to aid in confirming hypothyroidism: Free T4 (fT4) Endogenous thyroid-stimulating hormone (TSH) Possible thyroglobulin autoantibodies (TgAA)
When thyroid hormone levels are low due to primary hypothyroidism, most (around 75%) animals will have abnormally high cTSH levels. The antibodies (T3AA, T4AA, and TgAA) are markers for lymphocytic inflammation within the thyroid gland.
Serum Total T4 (TT4) Levels. A good screening test. TT4 will be low in most hypothyroid dogs,. i.e., the test has high sensitivity. The overlap between euthyroid and hypothyroid dogs can however be substantial - especially in the presence of concurrent disease i.e., low specificity.
Total T4 (or Thyroxine) measurement is a useful screening test for diagnosing hypothyroidism in dogs. It is also an economical way of following post pill treatment. Total T3 testing (or Triiodothyronine) is less helpful, but can indicate hypothyroidism.
