Abstract
To explore the accuracy of estimated 24-h urinary iodine excretion (24-h UIEest) in assessing iodine nutritional status. Fasting venous blood, 24-h and spot urine samples were collected during the day. The urinary iodine concentration (UIC) and urinary creatinine concentration (UCrC) were measured, and the urinary iodine-to-creatinine ratio (UI/Cr), 24-h UIEest, and 24-h urinary iodine excretion (24-h UIE) were calculated. At the population level, correlation and consistency between UIC, UI/Cr, 24-h UIEest and 24-h UIE were assessed using correlation analysis and Bland-Altman plots. At the individual level, receiver operating characteristic (ROC) curves were used to analyse the accuracy of the above indicators for evaluating insufficient and excessive iodine intake. The reference interval of 24-h UIEest was established based on percentile values. Indicator can accurately evaluate individual iodine nutrition during pregnancy remains controversial. Pregnant women (n 788). Using 24-h UIE as standard, the correlation coefficients of 24-h UIEest from different periods of the day ranged from 0·409 to 0·531, and the relative average differences ranged from 4·4 % to 10·9 %. For diagnosis of insufficient iodine intake, the area under the ROC curve of 24-h UIEest was 0·754, sensitivity and specificity were 79·6 % and 65·4 %, respectively. For diagnosis of excessive iodine intake, the area of 24-h UIEest was 0·771, sensitivity and specificity were 66·7 % and 82·0 %, respectively. The reference interval of 24-h UIEest was 58·43-597·65 μg. Twenty-four-hour UIEest can better indicate iodine nutritional status at a relatively large sample size in a given population of pregnant women. It can be used for early screening at the individual level to obtain more lead time for pregnant women.
Highlights
Correlation analysis of spot urinary iodine concentration, urinary iodine-to-creatinine ratio and estimated 24-h urinary iodine excretion with 24-h urinary iodine excretion Compared with 24-h UIE measurements, the correlation coefficients for spot UIC ranged from 0·324 to 0·481 (P < 0·01), those for UI/Cr were 0·303–0·454 (P < 0·01), and those for 24-h UIEest were 0·409–0·531 (P < 0·01)
Consistency of urinary iodine concentration, urinary iodine-to-creatinine ratio and estimated 24-h urinary iodine excretion with 24-h urinary iodine excretion at each time point Using 24-h UIE as the gold standard, we found that the relative average differences in 24-h UIEest between different periods during pregnancy clearly distinguished UIC and UI/Cr for the same periods
For the diagnosis of insufficient or excessive iodine intake, the AUC of 24-h UIEest showed no difference from UIC at fasting; there were differences with UI/Cr at fasting with P-values of 0·007 and 0·033, respectively
Summary
Study participants From June 2018 to October 2020, pregnant women were screened in hospitals in Harbin, Heilongjiang Province, an iodine adequate area in China. We used 1·8-ml cryogenic test tubes to place spot urine samples and directly transported the urine samples to the laboratory for analysis. External reference samples were provided by the National Laboratory for Prevention and Treatment of Iodine-Deficient Disorders in China. The above reference ranges were recommended by the guidelines for diagnosis and treatment of thyroid disorders in pregnancy and postpartum (2019)(5). The reference ranges of FT3, thyroglobulin antibody and thyroid peroxidase antibody (IT3000, Roche) were 3·1–6·8 pmol/l, 0-115 U/ml and 0–34 U/ml throughout pregnancy, respectively. According to the recommendation of the 2014 guidelines for the diagnosis and treatment of Fe deficiency and Fe deficiency anaemia during pregnancy[18] in China, the diagnostic criteria for Fe deficiency anaemia are serum ferritin level < 20 g/l and Hb level < 110 g/l. The significance levels quoted are two-sided and P < 0·05 was used to show statistically significant differences
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