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.