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.

Keywords: 131I; hyperthyroidism; iodinated contrast; iodine clearance; radioactive iodine; urinary iodine.

FIGURE 1

Mean (±SE) iodine‐to‐creatinine ratio (ICR) as a function of time after injection of Iopamidol in all cats. Day 0 represents a mean of the baseline values collected once daily for 72 hours before injection. Iopamidol was administered on Day 0.

FIGURE 2

Mean (±SE) urinary creatinine as a function of time after injection of Iopamidol in all cats. Day 0 represents mean of baseline urinary creatinine collected once daily for 72 hours before injection. Iopamidol was administered on Day 0.

FIGURE 3

Mean (±SE) urine specific gravity as a function of time after injection of Iopamidol in all cats. Day 0 represents a mean of the baseline USG collected once daily for 72 hours before to injection. Iopamidol was administered on Day 0.