Objectives: In cats with concurrent hyperthyroidism and non‐thyroidal illnesses such as chronic kidney disease, total thyroxine concentrations are often within the laboratory reference range (19 to 55 nmol/l). The objective of the study was to determine total thyroxine, free thyroxine and/or thyroid‐stimulating hormone concentrations in cats with mild chronic kidney disease.

Methods: Total thyroxine, free thyroxine and thyroid‐stimulating hormone were measured in three groups. The hyperthyroidism‐chronic kidney disease group (n=16) had chronic kidney disease and clinical signs compatible with hyperthyroidism but a plasma total thyroxine concentration within the reference range. These cats were subsequently confirmed to be hyperthyroid at a later date. The chronic kidney disease‐only group (n=20) had chronic kidney disease but no signs of hyperthyroidism. The normal group (n=20) comprised clinically healthy senior (>8 years) cats.

Results: In 4 of 20 euthyroid chronic kidney disease cats, free thyroxine concentrations were borderline or high (≥40 pmol/l). In the hyperthyroidism‐chronic kidney disease group, free thyroxine was high in 15 of 16 cats, while thyroid‐stimulating hormone was low in 16 of 16 cats. Most hyperthyroidism‐chronic kidney disease cats (14 of 16) had total thyroxine greater than 30 nmol/l, whereas all the chronic kidney disease‐only cats had total thyroxine less than 30 nmol/l.

Clinical Significance: The combined measurement of free thyroxine with total thyroxine or thyroid‐stimulating hormone may be of merit in the diagnosis of hyperthyroidism in cats with chronic kidney disease.