Update on diagnosis of canine hypothyroidism.

Until a clinically useful canine TSH assay is available, a valid test of thyroid hypofunction should (1) take into consideration valid (or at least predictive) measurement of the free T4 concentration in order to factor out the binding effects of drugs and nonthyroidal illness, and (2) should assess thyroid functional reserve. A validated equilibrium dialysis measurement of free T4 and the TSH stimulation test, provide the most useful diagnostic combination of tests now available. A therapeutic trial with L-thyroxine, despite being relatively safe, should be reserved once other diagnoses have been ruled out, and should be reevaluated to assure a clinical response which can be attributable to the medication.