Feline hyperthyroidism is a common endocrine disorder. A single dose of 148 MBq (4 mCi) 131I is 95–98% effective for the treatment of hyperthyroidism in cats; however, the cause for treatment failures has not been determined. In a series of 113 hyperthyroid cats having pertechnetate thyroid scintigraphy before treatment using a standard 148 MBq (4 mCi) 131I dose, the thyroid to salivary gland (T:S) ratio and the thyroid to background (T:B) ratio were calculated. Results in 107 (95%) cats successfully treated were compared with results in six (5%) cats that remained hyperthyroid after treatment. T:B ratio was significantly higher for cats that had treatment failure (median 13.0, range 3.6–73.0) than for cats successfully treated (median 4.4, range 1.2–69.0) (P=0.02), whereas there was no significant difference in their T:S ratios (P=0.2). The T:B ratio is a new approach to evaluating the thyroid pertechnetate scan with the intent of identifying which hyperthyroid cats may fail treatment using a standard 148 MBq (4 mCi) 131I dose and which, therefore, require a higher dose.