Assessing Degree of Hyperthyroidism in Cats

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The thyroid hormones, thyroxine (T4) and 3,3′,5- tri-iodothyronine (T3), are known to be tightly bound by plasma pro- teins. It is widely accepted that thyroid hormone activity relates to the portion of the T4 or T3 that is not bound. In human beings, the unbound T4 is approximately 0.025% of total T.., and the unbound T3 is approximately 0.3% of total T3. We have demonstrated that normal cats, com- pared with normal human beings, have less avid serum protein binding of T4 and T3. This undoubtedly is responsible for the fact that normal circulating total T4 and total T3 concentrations are lower in cats than in human beings.

In human subjects with hyperthyroidism, the plasma protein binding of T4 and T3 often is decreased, compared with eu- thyroid subjects.  Because of this decrease, degree of hyperthyroidism may be even greater than measurement of total T4 and total T3 alone would indicate. For this reason, when assessing the degree of hyperthyroidism in human beings, total T4 or total T3 measurements usually are combined with some type of assay of plasma protein binding.

To determine whether there is a similar decrease in relative plasma protein binding in feline hyperthyroidism, we studied plasma samples from 6 hyperthyroid cats, assayed simulta- neously with plasma samples from 5 euthyroid cats. Equilib- rium dialysis was performed by the method of Sterling and Brenner, which involved use of a 1: 20 dilution of serum enriched with radioactive T.. and T3 and dialyzed at physiologic tempera- ture and pH. The binding characteristics of the plasma samples from the hyperthyroid cats did not differ from those of the normal cats. Dialyzable T4 was 0.056 ± 0.006% (mean ± so) in the hyperthyroid cats, compared with 0.057 ± 0.009% in the euthyroid cats. The corresponding values for T3 were 0.46±0.09% and 0.47 ± 0.08%.

Since the unbound percentage of total T4 and of total T3 appears to be unchanged in cats with hyperthyroidism, it probably is justifiable to infer the degree of hyperthyroidism in these animals directly from measurements of total T4 and total T3 without also assessing the plasma T4 and T3 binding characteristics.