Absorbed thyroid dose and effective half-life were determined in 46 hyperthyroid 
cats after treatment with a low dose (mean 111MBq) of radioiodine intravenously. 
Thirteen of these cats had received iohexol for glomerular filtration rate (GFR) 
measurement within 24h before treatment with radioiodine in view of another 
ongoing study at our institution. Pre-therapy values were obtained for total 
thyroxine (TT(4)) and for the thyroid to salivary gland ratio with sodium 
pertechnetate gamma-camera imaging. All cats underwent post-therapy scans at 24, 
48 and 120 h for evaluation of radioactive iodine uptake (RAIU) and the 
effective half-life of radioiodine. The absorbed dose was calculated from the 
cumulative activity with Olinda software. Both groups were comparable in age, 
TT(4) and the ratio of thyroid activity to salivary gland activity. Statistical 
analysis revealed a significant decreased absorbed dose in the thyroid in the 
iohexol group. This decreased uptake was not accompanied by an decreased 
effective half-life of the radioiodine. The variation of inter-individual RAIU 
decreased in this group and more homogenous absorbed doses were obtained. No 
significant difference in outcome could be demonstrated. However, a tendency 
towards a higher number of residual hyperthyroidism in the iohexol group was 
noted (15 versus 6% in control group). This study demonstrates that iohexol 
interferes with the uptake of radioiodine in the hyperthyroid cat but does not 
provoke increased turnover. In this study, albeit including a small number of 
cats, outcome did not seem to be significantly affected.