![]() The minimum time between contrast exposure and RAI administration is not well established. In addition, the benefit of contrasted CT scans sometimes outweighs the problem of RAI interference, such as in the identification of cervical and mediastinal disease that cannot be imaged by sonography. Patients may also present for evaluation of thyroid nodules identified incidentally on a contrasted CT scan performed for another indication. The inorganic iodine introduced in the contrast bolus can also compete with radioactive iodine (RAI) for uptake and reduce the efficacy of diagnostic or therapeutic radioiodine use in patients with, for example, differentiated thyroid carcinoma. 1 Renal iodine clearance rate is not influenced by iodine intake it is not adaptive and not saturable. This causes a compensatory increase in TSH level, but this effect generally does not increase iodine levels beyond normal limits except in some geriatric patients or patients with other risk factors like hyperthyroidism, who may develop iodine-induced thyrotoxicosis. Administration of large doses of iodine and contrast medium causes a transient decrease in thyroid hormone synthesis through the Wolff-Chaikoff effect. In healthy patients, iodine diffuses into extracellular spaces and then follows 2 competitive pathways: uptake by the thyroid gland or excretion in urine. (To convert iodine to nanomoles per liter, multiply by 7.880.) Deiodination of the contrast medium molecules in the body can further introduce 0.01% to 0.15% more free inorganic iodine. The content of free iodide, according to manufacturer regulations, is far less than the total amount of organically bound iodine, with an upper limit of 50 000 μg/L, but this still means a single contrast-enhanced CT scan gives an adult over 30 times the minimum daily allowance of dietary iodine (5000 μg vs 150 μg). A typical chest CT study uses about 100 mL of intravenous contrast material, which translates to about 30 g of iodine. Radiographic studies such as computed tomography (CT) scans are commonly performed with intravenous contrast, which contains an enormous amount of iodine. This alleviates concerns about contrast use in patients with thyroid carcinoma interfering with adjuvant radioiodine therapy. The practice at our institution is to wait 2 months and then check a 24-hour urinary iodine level. ![]() Age, sex, weight, and estimated glomerular filtration rate were not significant.Ĭonclusions and Relevance These results may be used for guidance on the timing of RAI use following contrast exposure. Baseline iodine level was a significant predictor of postcontrast iodine levels. 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. 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). Intervention Morning urine samples were taken before the scan for analysis and then every 2 weeks thereafter for 12 weeks. ![]() Objective To better characterize how long it takes for the iodine load from an intravenous contrast bolus to clear from the body.ĭesign, Setting, and Participants 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. There is a paucity of literature on the minimum interval between contrast administration and RAI therapy. This can delay subsequent use of radioactive iodine (RAI) therapy because the iodine can compete for uptake. Importance Patients who undergo radiographic studies with contrast receive an enormous bolus of iodine.
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