Variability of iodine concentrations in the human placenta

Abstract

Iodine is an essential trace element, necessary for the production of thyroid hormones, which play a key role in optimal foetal growth and (neuro-) development. To date, iodine deficiency remains a health burden in many countries. We investigated the variability of placental iodine concentrations within and between individuals. We used 20 mother-neonate pairs from the ENVIRONAGE birth cohort, took samples at three standardized locations of the placentas, pooled and digested them, and determined the iodine concentrations using an ICP-MS method as an alternative for the Sandell-Kolthoff method. The variability between and within the three sample regions was calculated using the intra-class correlation coefficient (ICC) from the variance components of mixed models. With the Friedman test, the differences between placental biopsies were assessed. The ICC showed a higher between-placenta (68.6%) than within-placenta (31.4%) variability. Subsequently, we used our optimized method to determine iodine concentrations in 498 mother-neonate pairs, which averaged 26.1 μg/kg. For 96 mothers, the urinary iodine concentrations were also determined, which showed no correlation with the placental iodine storage, as was expected. Future studies are necessary to explore the effects of these placental iodine concentrations in relation to health outcomes of mother and child at birth and later in life.

Figure 1

Iodine levels of three biopsy regions of twenty randomly selected placentas. In Panel (A) the placental iodine concentrations of the three regions of each placenta (ID 1 to 20) are plotted. The min-max range was 18.4–43.0 μg/kg, and the 25^th, 50^th, and 75^th quartiles are 26.0, 28.3, and 31.6 μg/kg, respectively. In Panel (B) the 20 iodine concentration values per biopsy region are shown. The median values for the regions 1, 2, and 3 were 28.0, 29.2, and 30.0 μg/kg, respectively. There was no statistically significant difference between the regions (${\chi }_3^2$ statistic = 2.1; p = 0.35).

Figure 2

Histogram of the normally distributed placental iodine concentrations in the 498 participants of the ENVIRONAGE birth cohort. Gaussian distribution with a robust fit is shown in red. The lowest and highest observed iodine concentrations were 12.4 and 42 μg/kg, respectively. The median concentration was 26.0 μg/kg.

Figure 3

The largest umbilical cord artery on the foetal side of the placenta was used as reference location to identify the placental entry. Three biopsies of 2 by 2 cm were taken at a distance of 2 cm from the umbilical cord and treated further (removal of membranes and excess blood) to obtain three samples of each placenta.