When hyperthyroidism is treated with radioiodine, up to 75 per cent of the injected dose is excreted in the faeces and urine, which poses hazards to handlers. Three groups of hyperthyroid cats were treated with 120, 150 and 200 megabecquerel (MBq) of radioiodine, and samples of faeces and urine-soaked litter (USL) were collected over a 24-hour period, once a week, for four weeks. The amount of radioactivity in each homogenised sample was then measured using a sodium iodide detector. Radioactivity significantly decreased in both the faecal and USL samples over the first three weeks. Regardless of dose, there was no significant difference in faecal and USL samples between weeks 3 and 4. Faecal radioactivity was generally higher than the USL, but both were variable between cats at each time point. There were some significant differences in radioactivity between doses at various time points, but these were very small compared with the differences between time points. From the results, the maximum likely exposure to a worker or owner handling the waste was calculated. For cats treated with up to 200 MBq, radioactivity levels after two weeks were such that the waste could be designated as ‘very low level waste’ (a UK statutory definition) for disposal purposes.