| Abstract: | Objective The success of a low‐iodine diet (LID) is best determined by measurement of 24‐h urine iodine (U‐I) excretion. The aim of this study was to determine reliable estimates for 24‐h U‐I based on spot‐urine samples and to provide cut‐offs to determine the effectiveness of LID preparation. Design We prospectively measured iodine levels in 193 patients based on 24‐h‐ and spot‐urine samples before radioactive iodine therapy. The iodine was expressed as the 24‐h U‐I excretion (μg/day) and as two different indices from spot urine, simple iodine concentration (simple I) and the iodine/creatinine (I/Cr) ratio. Poor LID preparation was defined as I excretion of >150 μg/day according to the 24‐h U‐I measurement. Results The measured 24‐h U‐I was significantly higher than the two indices from spot urine (P < 0·001). However, there were statistically significant correlations between the 24‐h U‐I values and the two spot‐urine‐based indices; the correlation coefficient was 0·539 for simple I and 0·773 for I/Cr ratio (P < 0·001). The cut‐off of I/Cr ratio for poor LID preparation was >66·2 μg/g Cr (sensitivity 96·4%, specificity 83·6%, positive predictive value 50·0% and negative predictive value 99·3%). Conclusions We demonstrated that the I/Cr ratio from spot urine could serve as a useful and reliable alternative to 24‐h urine collection as it has acceptable diagnostic values for detecting poor LID preparation. |
