Urinary iodine excretion after contrast computed tomography scan: implications for radioactive iodine use

Patients who undergo radiographic studies with contrast receive an enormous bolus of iodine. This can delay subsequent use of radioactive iodine (RAI) therapy because the iodine can compete for uptake. There is a paucity of literature on the minimum interval between contrast administration and RAI therapy.

To better characterize how long it takes for the iodine load from an intravenous contrast bolus to clear from the body.

A prospective cohort of 21 adults undergoing intravenous contrast CT studies at a tertiary academic medical center; exclusion criteria included history of thyroid disease or thyroidectomy, history of renal insufficiency, pregnancy, and other contrast administration within 1 year.

Morning urine samples were taken before the scan for analysis and then every 2 weeks thereafter for 12 weeks. RESULTS The median baseline iodine level was 135 μg/L (range, 29-1680 μg/L), and median peak level was 552 μg/L (range, 62-6172 μg/L). Median time for urinary iodine level to normalize was 43 days, with 75% of subjects returning to baseline within 60 days, and 90% of subjects within 75 days. Baseline iodine level was a significant predictor of postcontrast iodine levels. Age, sex, weight, and estimated glomerular filtration rate were not significant.

These results may be used for guidance on the timing of RAI use following contrast exposure. The practice at our institution is to wait 2 months and then check a 24-hour urinary iodine level. This alleviates concerns about contrast use in patients with thyroid carcinoma interfering with adjuvant radioiodine therapy.