Hyperthyroidism can be difficult to diagnose in cats with renal disease since the total thyroxine (tT4) concentration is frequently within the laboratory reference range. This study investigated whether the measurement of thyroid stimulating hormone (TSH) could aid in diagnosing hyperthyroidism in cats with chronic renal failure (CRF). TSH was measured using a chemiluminescent assay (lmmulite TSH assay, Diagnostic Products Corporation) developed for use in the dog. TSH and tT4 were measured in plasma samples from 17 hyperthyroid and 17 euthyroid cats, which had all been diagnosed with CRF by the demonstration of an elevated plasma creatinine concentration (> I77 mmol/I) in conjunction with an inappropriately low urine specific gravity (< 1.035). The plasma samples selected from the hyperthyroid cats were taken when there was a clinical suspicion of hyperthyroidism but when a tT4 concentration was within the reference range. The diagnosis of hyperthyroidism was confirmed by documenting an elevated tT4 concentration or by a T3 suppression test. The euthyroid cats had no clinical signs of hyperthyroidism and all tT4 concentrations were within or below the normal range and remained so for a minimum of 6 months. A MannWhitney test was used to make statistical comparisons between the hyperthyroid and non-hyperthyroid cats. All the hyperthyroid cats had TSH levels equal to or below the limit of detection of the assay (0.03 ng/nl). Fifteen of 17 euthyroid cats had measurable TSH concentrations. The median TSH of the hyperthyroid cats (0.03 ng/ml) was significantly lower (p<0.00I) than the median TSH concentration of the euthyroid cats (0.05 ng/ml; range 0.03-0.11 ng/ml), The tT4 concentration was in the lower half of the laboratory reference range (<27nmol/l) in all the euthyroid cats. The combined diagnostic criteria of a high tT4 (>27nmol/l) and a low TSH concentration (<0.03nglml) provided a sensitive and specific test for hyperthyroidism in cats with CRF in this study. Further investigation of TSH measurement is now required using a larger population of cats. Additionally an assay should be developed with improved sensitivity for felineTSH.