Thyroid Testing Interpretation

The following table contains possible interpretations of various thyroid states for dogs, cats, and horses.


Measurement of free-T4 may be useful in differentiating dogs with true hypothyroidism from dogs with reduced thyroid function due to other chronic illness (e.g., hyperadrenocorticism, liver disease, diabetes), drug administration (e.g., glucocorticoids), and malnutrition. 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 free-T4 test may be a useful additional test in cats that are clinically hyperthyroid, but have normal total T3 and/or total T4 concentrations. Cats with a low total protein due to liver or kidney disease may have falsely elevated free T4 levels.


Horses seldom have primary hypothyroidism, but often have hypothyroidism secondary to equine metabolic syndrome (EMS) or pituitary pars intermedia dysfunction (PPID, Cushing’s syndrome). A few true hyperthyroid horses have been documented. Supplements high in iodine or containing kelp may cause high thyroid levels and occasionally signs of hyperthyroidism.

Anti-inflammatories, steroids, chronic illness, dietary supplements, and a variety of other factors may affect thyroid function. Taking into consideration the animal's history and results of the physical examination and other laboratory tests, possible interpretations are as follows:

Total T4 Total T3 Free-T43,4 TSH1 Possible Interpretation
low low low normal/high primary hypothyroid
low normal/low normal/high normal sick euthyroid
low normal/high low normal primary hypothyroid (early stage)
low high low normal/high T3 autoantibody & hypothyroid
low normal/low low normal secondary hypothyroidism
normal normal normal normal euthyroid
normal low normal normal/high euthyroid
normal normal low normal/high not diagnostic for hypothyroidism
high normal/low high normal T4 autoantibody/T4 supplementation
high normal/low low normal/high T4 autoantibody & hypothyroid
high high high normal functional thyroid tumor
high high low normal/high T3 & T4 autoantibody; hypothyroid

Special consideration:

  1. TSH testing is available only for dogs and cats.
  2. TgA (thyroglobulin autoantibody) testing is only available for dogs.
  3. Free T4 is available by Immulite or by equilibrium dialysis for dogs. TgA positive dogs may have falsely elevated Free T4 Immulite results, so should always be tested using the free T4 by dialysis method available for other species.
  4. Free T4 by equilibrium dialysis is the only Free T4 test method we use for cats and horses.