EN04 - Urinary Iodine Clearance Following Iodinated Contrast Administration in Normal Cats; Implications for 131I Use

Abstract: Background- Exogenous iodine sources interfere with uptake of radioactive iodine (¹³¹I) by the thyroid gland. This has implications for the treatment of cats with hyperthyroidism that have recently undergone computed tomography (CT) with intravenous (IV) iodinated contrast medium (ICM). In human medicine, a 4-week delay is required between CT and ¹³¹I.

Hypothesis/Objectives- It will require 4 weeks for urinary iodine concentration (UIC) to decrease to baseline after IV ICM administration.

Animals- 10 healthy adult neutered male cats living in a group colony.

Methods- Cats were sedated and received IV ICM (600 mg Iodine/kg). On samples collected prior to injection (n=3) and on days 1, 2, 3, 7, 10 and weeks 2-6 post injection, UIC and urinary creatinine concentration (UCC) were measured, and the urinary iodine creatinine ratio (ICR) was calculated. Urinary outcome variables were modelled using a linear mixed effects model with time as a fixed effect and individual cats as a random effect.

Results- UIC increased 37- to 884-fold on day 1 post ICM injection and returned to baseline during week 2 of the study. Compared to baseline, UIC was significantly increased for days 1-7, (all p< 0.001); UCC was significantly lower for days 1-10 (all p< 0.029); and ICR was significantly increased from day 1-10 (all p< 0.001, except day 10 p=0.049).

Conclusions- Urinary clearance of iodine after IV ICM administration requires between 7-14 days to return to baseline in healthy cats. This suggests that a 2-week delay between the iodinated contrast study and ¹³¹I treatment is appropriate.