Researchers have provided new evidence that could act as a guide for the use of radioiodine (RAI) therapy after patients have undergone radiographic imaging studies that use iodine as a contrasting agent.
A wait of around 75 days after such scans should be enough to ensure adequate clearance of inorganic iodine from the body before the administration of RAI, say Nitin Pagedar (University of Iowa Hospitals and Clinics, Iowa City, USA) and colleagues.
Patients who undergo radiographic studies with contrast receive a large bolus of inorganic iodine, which can compete with RAI for uptake and reduce the efficacy of RAI used to treat patients who have, for example, differentiated thyroid carcinoma.
So far, the minimum time between contrast exposure and RAI administration has not been well established, although the American Thyroid Association Guidelines for the Management of Differentiated Thyroid Carcinoma recommend that "iodinated contrast should be avoided if RAI therapy is planned within the subsequent few months," note Pagedar et al.
In the current study, 21 patients undergoing intravenous contrast CT studies provided a baseline urine sample on the morning of their scan. Additional morning samples were collected every 2 weeks for 12 weeks and analyzed for iodine levels. The time for each patient's iodine level to return to the baseline, pre-CT level was recorded.
As reported in JAMA Otolaryngology - Head and Neck Surgery, the median baseline iodine level was 135 g/L (range: 29-1680 g/L), while the median peak iodine level was 552 g/L (range: 62-6172 g/L).
Kaplan-Meier analysis showed that the estimated median time to baseline, pre-CT iodine level was 43 days, with 75% returning to baseline inside 60 days and 90% within 75 days.
"These results may be used for guidance on the timing of RAI use following contrast exposure," say Pagedar and team.
The researchers say that the practice at their institution is to wait 2 months after radiographic imaging of patients before measuring their 24-hour urinary iodine levels.
"However, other practitioners may elect to risk delaying therapy and wait a longer period (eg, 75 days) so that 90% of patients would be expected to have adequate iodine levels, which carries the benefit of eliminating the expense and effort of repeated testing," suggests the team.
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