Treatment of feline hyperthyroidism (HTH) is frequently associated with the unmasking of chronic renal failure (CRF). It is generally assumed that cats that develop azotaemia following treatment for HTH have a worse prognosis than cats that do not, and this can affect treatment recommendations given to clients. The aim of the present study was to test this assumption.

Data from cats diagnosed with HTH between May 1995 and December 2004 were included in the study. Treatment was by surgical thyroidectomy or oral anti-thyroid medication. Cats were included in the study if they did not have evidence of azotaemia prior to, or at the time of, diagnosis of HTH and if treatment of HTH was successful as defined by at least one total thyroxine concentration within reference range (less than 55 nmol/l). Cats were assigned to two groups dependent on their renal staus post-treatment. Cats in Group 1 developed azotaemia (plasma creatinine concentration greater than 177 umol/l)within 6 months of normalization of thyroid function while those in group 2 did not develop azotaemia after treatment. Cats were censored if they were still alive at study completion or if lost to follow up. Data are presented as median (range). Log-rank analysis was performed to compare the survival of the two groups. Creatinine concentrations were compared using a Mann-Whitney test.

One hundred and sixteen cats met the inclusion criteria for the study; 27 in Group 1 (4 censored) and 89 in Group 2 (25 censored). Creatinine concentrations in Groups 1 and 2 were significantly different both before [124 (62-171) versus 95 (42-173) umol/l; p = 0.005] and after [205 (178-383) versus 134 (71-176) umol/l; p <0.001] treatment. There was no sugnificant difference in the median survival times of teh 2 groups (p = 0.82). Median survival time in the cats that died was 595 (62-2016) and 584 (29-2044) days in groups 1 and 2 resoectively. Median survival time in the censored cats was 409 for Group 1 and 532 days for Group 2.

The median survival times reported in this study are similar to previously published data derived from animals treated with radioactive iodine, HTH cats which developed azotaemia after treatment did not appear to have a worse prognosis than than cats which remained non-azotaemic; therefore, the appearance of mild stable CRF should not affect treatment advice given to clients. In particular, it is not recommended to withdraw treatment for HTH in azotaemic cats to normalise the creatinine concentration.