Hyperthyroidism is a common endocrine disorder of older cats. Clinicopathologic alterations that may be associated with hyperthyroidism include polycythemia in ~50% of affected cats, increased serum activity of alkaline phosphatase (ALP) and alanine aminotransferase (ALT) in ~50-60% of cats. Serum creatinine may be increased. Treatment of hyperthyroidism may result in exacerbation of azotemia in some patients, presumably related to decreased cardiac output as the disease becomes controlled.

Serum total T4 is the screening test of choice for hyperthyroidism, and is increased in the majority of cats with the disease. Occasionally, cats with early hyperthyroidism, or those with significant non-thyroid illness may have total T4 within the upper half of the reference interval (i.e., 25-50 nmol/L). Options for additional testing in those circumstances include:

• Measurement of free T4 by equilibrium dialysis- this test identifies most cats with mild hyperthyroidism or nonthyroidal illness, but has a false positive rate of ~ 5-6%. Free T4 should be interpreted in the context of clinical signs and serum total T4 concentration.
• Repeated total T4 measurement as serum T4 may fluctuate.
• Do not collect blood into SST collection tubes for thyroid hormone analysis.

• T3 suppression test- serum T4 concentration is measured before and after administration of timed doses of exogenous T3. In euthyroid cats, the serum T4 concentration decreases by at least 50%. Hyperthyroid cats show little or no change in serum T4 concentration.