Measurement of serum concentrations of free thyroxine, total thyroxine, and total triiodothyronine in cats with hyperthyroidism and cats with nonthyroidal disease

Objective-To determine the usefulness of measuring serum free thyroxine (T- 4) concentration as a diagnostic test for hyperthyroidism in cats, and to d etermine the influence of nonthyroidal disease on free T-4 concentration in cats without hyperthyroidism. Design-Prospective case series. Animals-917 cats with untreated hyperthyroidism, 221 cats with nonthyroidal disease, and 172 clinically normal cats. Procedure-Serum free T-4, total T-4, and total triiodothyronine (T-3) conce ntrations were measured in cats with untreated hyperthyroidism and cats wit h nonthyroidal disease. Serum total T-4 and T3 concentrations were determin ed by use of radioimmunoassay, and free T-4 concentration was measured by u se of direct equilibrium dialysis. Reference ranges for hormone concentrati ons were established on the basis of results from the 172 clinically normal cats. Results-Sensitivity of serum free T-4 concentration as a diagnostic test fo r hyperthyroidism was significantly higher than the test sensitivity of eit her total T-4 or T-3 concentration. Of the 221 cats with nonthyroidal disea se, 14 had a high free T-4 concentration (ie, false-positive result). There fore, calculated specificity of measuring serum free T-4 concentration as a diagnostic test for hyperthyroidism was significantly lower than test spec ificity of measuring either the total T-4 or T-3 concentration. Conclusions and Clinical Relevance-Results indicate that determination of f ree T-4 concentration is useful in the diagnosis of hyperthyroidism, especi ally in cats in which hyperthyroidism is suspected but total T-4 and T-3 co ncentrations are within reference ranges. However, because some cats with n onthyroidal disease have high serum free T-4 concentrations, hyperthyroidis m should not be diagnosed solely on the finding of high free T-4 concentrat ion.