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However, 13% to 38% of hypothyroid dogs have normal TSH concentrations, 10-12,25 so a normal TSH concentration does not exclude the diagnosis. Because of this limitation, it is often helpful to evaluate fT4 and TSH simultaneously as confirmatory tests. If the fT4 is low, a diagnosis of hypothyroidism can be made.
• 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.
Healthy dogs tend to have lower total T4 values (12–45 nmol/l) than healthy humans (40–128 nmol/l). Hence the assay must be sensitive enough to detect values less than 12 nmol/l to differentiate hypothyroid from euthyroid dogs.
If the total T4 concentration is well within the normal range, then your dog is not hypothyroid. If the total T4 concentration is at the low end or below the normal range, and your dog has supportive clinical signs, then hypothyroidism is likely.
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.
T4 and T3. 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.
