Not all hyperthyroid cats exhibit clinical signs, and concurrent illness may skew the laboratory assessment of thyroid function. Here’s the latest research on what tests to perform to establish a diagnosis. In this article, I describe the most common clinical signs and physical examination findings in hyperthyroid cats. I also review the available diagnostic methods, including their advantages and disadvantages. Hyperthyroidism is a common endocrine disorder in senior cats. Clinical signs in cats with hyperthyroidism appear to be declining in severity, likely because of earlier recognition of the disorder and the use of routine total T4 screening tests in senior cats. Nonthyroidal illness can interfere with the diagnosis of hyperthyroidism, especially in cats with mild increases in total T4. Measuring free T4 by equilibrium dialysis and performing nuclear scintigraphy can enhance our ability to accurately diagnose this disorder in cats, including cats with concurrent nonthyroidal illness.