iodinated contrast is frequently given to enhance the diagnostic utility of computed tomography ( ct ) . the contrast material results in a large increase in body iodine stores , which is progressively cleared over several weeks to months . a typical ct study uses about 100  cc of intravenous contrast material , which translates to about 30  g of iodine . there is concern that this transient increase in iodine can compete with i and interfere with subsequent management , such as whole - body scans and treatment with radioactive iodine in thyroid cancer patients . in addition , many groups recommend iodine depletion diets to optimize the therapeutic response to radioactive iodine in these patients [ 2 , 3 ] . thus , avoidance of iodinated contrast material is particularly important as it can lessen the effectiveness of radioactive therapy [ 4 , 5 ] . the duration of time required for body iodine stores to return to normal following iodinated contrast administration has not been well studied in thyroid cancer patients . recommendations for avoidance of radioiodine administration after iodinated contrast vary from 4 weeks to 1 year , with most suggesting about 3 months . iodinated contrast should be avoided if radioactive iodine therapy is planned within the subsequent few months  . the european consensus guidelines also state that radioactive iodine administration should be postponed for two to three months after the event of iodine contamination . the normal thyroid has a considerable storage pool of organified iodine   and since the body recirculates this iodine , thyroidectomised patients may possibly have a quicker elimination time than euthyroid individuals . if confirmed , then a briefer interval between iodinated contrast administration and subsequent radioiodine therapy could be considered and would expedite therapy . the objective of this study is to compare iodine clearance following iodinated contrast administration in thyroidectomised thyroid cancer patients and euthyroid individuals . a convenience study population was drawn from patients referred to the saint boniface general hospital diagnostic imaging department for iodinated contrast - enhanced ct studies . thyroidectomised thyroid cancer patients were notified of the study by members of the cancercare manitoba thyroid cancer disease site group who identified eligible patients requiring contrast - enhanced ct for clinical reasons . eligible thyroidectomised patients had previously undergone total or near - total thyroidectomy for thyroid cancer and were on a stable dose of suppressive l - thyroxine therapy . control euthyroid subjects were recruited from routine diagnostic ct referrals coded to receive iodinated contrast . eligible control individuals included those without documented thyroid disease and who were not taking thyroid hormone replacement . the university health research ethics board approved the study for the university of manitoba and signed and informed consent was obtained . exclusion criteria included pregnancy , other major sources of iodine including any history of amiodarone use ever or iodinated contrast in the preceding 12 months , treatment with lithium , abnormal renal function ( serum creatinine > 170  umol / l ) , inability or unwillingness to participate , and control subjects with known thyroid disease or those who were found to have abnormal thyroid function or anticipated survival of less than one year . routine ct protocols used the same dye load ( 100  cc of omnipaque 350 , ge healthcare ) for all adult patients undergoing scanning of the neck , chest , abdomen , or pelvis . we recorded routine demographics and clinical characteristics including sex , age , height , weight , and any iodine - containing medications , vitamin supplements , or herbal products . prior to the patient undergoing the iodinated contrast enhanced ct , a urine sample subjects were asked to collect a urine specimen every 2 weeks for the first 8 weeks and then monthly up to 6 months for 9 collections in total . urinary iodine concentration is currently the most practical biomarker for determining the iodine nutritional status [ 8 , 9 ] . approximately 90% of all iodine ingested in the diet will be excreted in the urine . urine samples were stored in a 60  c freezer until analysis at the end of the study . all samples were batch - run for assessment of urinary iodine in conjunction with creatinine at health canada . urinary iodine concentrations were determined using ammonium persulfate digestion followed by colorimetric analysis based on the sandell - kolthoff reaction according to a modified microplate method . urinary creatinine concentrations were determined by enzymatic idms - standardized two - point rate on an ortho - clinical diagnostics vitros 5 , 1fs analyser . the urinary iodine to creatinine ratio ( microg / g ) was calculated as the measure of iodine clearance   as used by the third national health and nutrition examination survey ( nhanes iii ) ( 19841994 ) . a review concluded that spot urinary iodine concentration is a reliable measure of the iodine intake in the population when expressed as a function of urinary creatinine to correct for the influence of fluid intake . demographic and clinical characteristics of the thyroidectomised thyroid cancer patients and of the euthyroid controls are described using medians , means , and standard deviations ( sd ) for continuous variables and frequencies and percentages for categorical variables . group comparisons for continuous data were conducted with the student 's t - test for specific time points . models based upon generalised estimating equations ( gee ) were used to analyse combined data for all time points from 4 weeks onwards assuming an exchangeable correlation structure . separate gee models were then constructed to look for an effect of time since contrast administration or a time  group interaction . the dependent variable used for the primary analysis was the urinary iodine creatinine ratio . in a secondary analysis we enrolled 6 thyroidectomised thyroid cancer patients and 7 control subjects . in total 50 samples from the thyroidectomised thyroid cancer group there were no missing collections from the control group ; there were 4 missing collections from the thyroidectomised thyroid cancer group . the mean urinary iodine creatinine ratio was 248.2  microg / g at baseline for the thyroidectomised thyroid cancer patients and 138.3  microg / g at baseline for the control patients . as anticipated , the two - week mean values were still elevated above baseline ( thyroidectomised thyroid cancer 770.3  microg / g versus 670.3  microg / g control groups ) . figure 1 shows the mean urinary iodine creatinine ratios for the two groups , demonstrating a return to baseline values 4 weeks following iodine contrast administration . pairwise testing showed no significant group differences in urinary iodine creatinine ratios at baseline or any subsequent time point . when expressed as the change from baseline , only the 2-week sample showed significantly increased urinary iodine clearance ( table 2 ) . gee was used to examine for differences in urinary iodine creatinine ratios between the two groups from week 4 onwards and no significant group effect was identified ( p = 0.53 ) . in a separate gee model we did not identify any significant time effect ( p = 0.19 ) or time  group interaction ( p = 0.84 ) . a gee model with change from baseline from week 4 onwards showed a statistically significant difference between the two groups , lower in the thyroidectomised thyroid cancer group ( parameter 113.1  microg / g [ 95% confidence interval 12.8 to 213.3  microg / g ] , p = 0.027 ) . following exposure to a large iodine load , the plasma concentrations of free iodine remain elevated as the body 's iodine stores are expanded in the interstitial fluids and in the thyroid colloid . current literature suggests that , in euthyroid individuals , the body 's iodine stores are increased for at least 3 months following iodinated contrast . our study finds that the urinary iodine creatinine ratio returns to baseline by 4 weeks following iodinated contrast in both euthyroid and thyroidectomised thyroid cancer subjects . in addition , there was no significant difference between the thyroidectomised and euthyroid patients at 4 weeks , nor throughout the rest of the study period up to 6 months . in which urinary iodine levels returned to baseline at one month in thyroidectomised thyroid cancer patients after receiving iodinated contrast . a prospective study by nimmons et al . showed that the median time for urinary iodine levels to normalize in euthyroid subjects was 43 days , with 75% of subjects returning to baseline by 60 days . our study enrolled only 13 subjects in total and may have been underpowered to detect significant differences between the two groups . the small numbers of cases and controls are compensated for by the multiple time points assessed , which allowed for all urinary collections from 4 weeks onwards to contribute to the statistical analysis using gee . in addition , thyroid cancer patients need to be on a low iodine diet prior to radioiodine treatment . while this study demonstrates that urinary iodine creatinine ratio returns to baseline by 4 weeks in both thyroidectomised thyroid cancer and euthyroid subjects , it does not look at the effect of a low iodine diet on the urinary iodine creatinine ratio post iodinated contrast . we did not collect data on noncontrast sources of exogenous iodine ( e.g. , diet , vitamin supplements ) , but our analysis of the change in iodine excretion from baseline partially compensates for interindividual differences in dietary iodine intake . increasingly , ct is being compared to ultrasound , magnetic resonance imaging , or positron emission tomography techniques . as more information is gained about the sensitivity , specificity , and accuracy of these techniques , ct may be used less frequently which would ameliorate the concern of iodinated contrast and therapeutic radioactive iodine . in conclusion , we found that 4 weeks may be sufficient for urinary iodine creatinine ratios to return to baseline value following iodinated contrast in both thyroidectomised thyroid cancer and euthyroid subjects . this suggests that clearance time may be more rapid than previously thought and that a shorter time interval between iodinated contrast and radioiodine therapy might be considered but requires confirmation in larger studies that include a broad case mix in terms of age and dietary iodine intake .
purpose . to compare iodine clearance following iodinated contrast administration in thyroidectomised thyroid cancer patients and euthyroid individuals . methods . a convenience population ( 6 thyroidectomised thyroid cancer patients and 7 euthyroid controls ) was drawn from patients referred for iodinated contrast - enhanced computed tomography ( ct ) studies . 
 subjects had sequential urine samples collected up to 6 months ( 50 samples from the thyroidectomised and 63 samples from the euthyroid groups ) . 
 t - tests and generalised estimating equations ( gee ) were used to test for group differences in urinary iodine creatinine ratios . 
 results . 
 groups had similar urinary iodine creatinine ratios at baseline , with a large increase 2 weeks following iodinated contrast ( p = 0.005 ) . 
 both groups had a return of urinary iodine creatinine ratios to baseline by 4 weeks , with no significant group differences overall or at any time point . 
 conclusions . 
 thyroidectomised patients did not have a significantly different urinary iodine clearance than euthyroid individuals following administration of iodinated contrast . 
 both had a return of urinary iodine creatinine ratios to baseline within 4 weeks .