Scase, Tim

Introduction and Objectives: Hyperthyroidism in cats, caused by nodular hyperplasia or adenomas, is clinically and histologically similar to toxic nodular goiter in humans. Subclinical hyperthyroidism in humans is defined as low thyrotropin (TSH) in conjunction with within-reference-range thyroid hormone concentrations, but has not previously been defined in cats. The objective of this study was to test the hypothesis that euthyroid senior cats with low TSH have histological evidence of thyroid nodular hyperplasia and/or adenoma. Design: Thyroid glands removed postmortem from four groups of cats (n = 73) were examined histologically and scored in a blinded fashion. Clinically euthyroid senior (>7 years) cats were divided into two groups dependent on their TSH concentration—TSH below the limit of quantification (LOQ) of the assay (<0.03 ng/mL; n = 15; UndetectableTSH group) and TSH above the LOQ (≥0.03 ng/mL; n = 31; DetectableTSH group)—using archived plasma samples, collected 0–6 months antemortem. Thyroids were also scored for two control groups: Young group (cats <6 years old; n = 13) and Hyperthyroid group (clinically and biochemically hyperthyroid cats; n = 14). Main outcome: Cats in the UndetectableTSH group had a higher frequency of nodules, a greater percentage of abnormal thyroid tissue, and a higher overall histopathological grade than cats with detectable TSH had. Conclusion: Euthyroid (as defined by total thyroxine) senior cats with low TSH are likely to have histological evidence of nodular thyroid disease, and such cats could be considered to be subclinically hyperthyroid.