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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.
• 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)
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.
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.
T4 levels can be measure 6–8 weeks after initiating treatment. Samples should be drawn just before tablet administration (trough) and 4–6 hours after administration (peak). Ideally levels of both peak and trough samples should be within the high and low normal range, respectively.
TSH is normal in many abnormal dogs, so we recommend measuring it in combination with other thyroid function tests. Thyroglobulin autoantibody (TgA) testing is recommended for dogs with a breed predilection for autoantibody problems or those with unexpectedly high T3 or T4 values.
The identification of a total T4 or free thyroxine below the reference range, along with an elevated thyrotropin (thyroid-stimulating hormone; TSH), provides the greatest specificity for diagnosis, although these results can also be seen in dogs with other diseases.
