J Am Vet Med Assoc 192[1]:7 Jan 1'88

In hyperthyroid humans there is decreased plasma protein binding of T4 and T3 resulting in an increased degree of hyperthyroidism than indicated by total serum T4 and T3 measurements.Thyroid hormone activity is related to the percent of unbound T4 and T3.
In a study of 6 hyperthyroid and 5 euthyroid cats the authors found no significant differences in the percentage of unbound T4 and T3 levels in hyperthyroid or euthyroid cats.
Thus measuring total T4 and T3 levels appears to correlate with the degree of hyperthyroidism in cats.