Urinary iodine clearance after iodinated contrast administration to healthy cats

Abstract Background Exogenous iodine interferes with the uptake of radioactive iodine (131I) by the thyroid gland. This has potential implications for the treatment of cats with hyperthyroidism that have recently undergone computed tomography (CT) with IV administration of iodinated contrast medium (ICM). Hypothesis To determine the time to normalize urinary iodine clearance after administration of ICM. We hypothesized that it would require 4 weeks for urinary iodine concentration (UIC) to decrease to baseline after IV administration of ICM. Animals Ten healthy adult neutered male cats. Methods All cats were sedated and received Iopamidol at a dose of 2 mL/kg (600 mg/kg). Urinary iodine and creatinine concentrations were measured before administration of Iopamidol and on days 1, 2, 3, 7, 10 and weeks 2 to 6 after administration. The urinary iodine‐to‐creatinine ratio (UICR) was calculated. Outcome variables were modeled using a linear mixed‐effects model. Results Urinary iodine concentration increased 37‐ to 884‐fold on Day 1 after ICM injection and returned to baseline during Week 2. Compared with baseline, UIC was significantly increased for Days 1 to 7 (all P < .001); UC was significantly lower for Days 1 to 10 (all P < .03); and UICR was significantly increased from Days 1 to 10 (all P < .001, except Day 10 P = .05). Conclusions Urinary clearance of iodine after IV administration of ICM requires 10 days to return to baseline in healthy cats. A 2‐week interval between the iodinated contrast study and 131I treatment could be appropriate but needs to be confirmed in hyperthyroid cats.