Received dose variability after administration of I-131 for treatment of hyperthyroidism in cats

Injectable radioactive iodine (I-131) frequently is used to treat hyperthyroidism in cats. In human medicine, residual activity after injection of radionuclides has been reported, and the actual quantity administered is recorded after administration.

Our aim was to evaluate actual administered dose variability after administration of preprepared I-131 single unit doses for the treatment of hyperthyroidism in cats.

Twenty-seven cats with hyperthyroidism treated with I-131 between April 2017 and March 2019.

Retrospective observational study of cats treated with preprepared single unit I-131 doses. For each dose, the measured activity before administration and residual activity were recorded. The measured dose and the actual dose administered were compared to the prescribed dose.

Measured activity before administration ranged from 88.4% to 103.3% of the prescribed dose. Mean residual activity was 5.2 ± 3.0 MBq (ranging from 1.5% to 15% of the prescribed dose). The actual dose administered (measured activity - residual activity) ranged from 79.1% to 100.2% of the prescribed dose. Seventeen of 28 (60.7%) of the actual administered doses differed between 10% and 20% of the prescribed dose. One administered dose had a >20% difference compared to the prescribed dose (79.10% of the prescribed dose).

Our study identified variability in the residual and actual administered activity of I-131 as compared to the prescribed dose, which should be taken into consideration when treating cats with (predrawn) I-131.