Graves T.K. and Peterson M.E.
As expertise among small animal practitioners grows, feline hyperthyroidism is being diagnosed earlier in the course of the disease. There are, in fact, a growing number of cats with clinical signs of hyperthyroidism and palpably large thyroid glands whose serum total thyroxine (T4) and triiodothyronine (T3) concentrations are within the normal or borderline range. This condition can be referred to as “occult” hyperthyroidism. Early detection and treatment of feline hyperthyroidism presents an obvious advantage in avoiding some of the deleterious effects of chronic thyroid hormone excess (eg, cardiomyopathy). Recent advances have been made in the diagnosis of occult hyperthyroidism in cats. It has been found, for instance, that serum thyroid hormone concentrations can fluctuate in and out of the normal range in some cats with hyperthyroidism. Recent work also has laid the groundwork for use of a T3 suppression test as an aid in the diagnosis of early, mild, or occult hyperthyroidism in cats. The purpose of this chapter is to discuss these and other developments, as well as to discuss some of the problems confronted in diagnosing occult hyperthyroidism in cats.