Recent estimates from surveys indicated that the median UIC (urinary iodine concentration) of the Indian population was 154 µg/L [4]. While iodine deficiency still persists in many parts of the country, iodine excess has recently been documented in a few studies from Delhi [5,6]. Thus, we aimed at assessing the correlation, if any, between MUIC and neonatal TSH levels, and to study the current iodine status in a group of mother-infant dyads.

This study was performed in a tertiary care center in New Delhi, India between November 2015 and November 2016. The study protocol was approved by Institutional Ethics Committee. Term healthy neonates weighing >2500 g and their mothers were included. Mothers who were hypothyroid, had autoimmune disease (diagnosed as anti TPO antibody positive), on antithyroid medication, requiring intensive postpartum care or had blood stained urine were excluded. The details of the age of mothers, parity, the type of salt used in their household were collected by verbal information. The antiseptic used during delivery was recorded.

Cut-offs and recommendations of the procedures to be followed were adopted from guidelines released by WHO [7]. According to WHO, the proportion of neonates with TSH values >5 mIU/L in whole blood is proportional to the degree of iodine deficiency during pregnancy. When a sensitive TSH assay is performed on the samples, <3% frequency of TSH values >5 mIU/L indicates iodine sufficiency in a population.

TSH was measured by time resolved fluro-immunoassay on the Genomic screening processor (Perkin Elmer Life Sciences, Turku Finland). The limit of detection of the assay was 2 µU/L and the coefficient of variation <5%. The iodine concentration in urine was measured by the Wet Digestion method with internal quality control, and the results were expressed as µg/L.

The sample size of 400 mother neonatal dyads was estimated with the power of 90% and alpha error of 5%, based on a previous study which established the correlation between neonatal TSH and MUIC (r = -0.67) [8].

The total number of deliveries in our institution during the study period was 12785. The number of neonates having TSH between 5.1-10 mIU/L was 1555 (12.1%). After the maternal and neonatal exclusion factors, 1050 neonates were found eligible. Out of these, urinary samples from a convenient sample of 400 mothers of cases and 144 mothers of controls were collected.

According to WHO, the proportion of neonates with TSH values >5 mIU/L in whole blood is proportional to the degree of iodine deficiency during pregnancy. Elevated neonatal serum TSH concentration may indicate insufficient supply of thyroid hormones to the developing fetal brain, and is therefore the only measure that allows prediction of brain damage due to iodine deficiency [2]. The cutoff of neonatal TSH >5 mIU/L used in our study has been suggested to be a good discriminator for severe iodine deficiency in the population; though, reports from certain areas of mild deficiency suggest it to be questionable.

The reasons for elevation of TSH in iodine deficiency are implicit. However, elevation of neonatal TSH levels in iodine excess group is contrary to the popular belief; however, it could be explained by a mechanism called Wolff-Chaikoff effect [9]. Both iodine excess and deficiency causing a raised neonatal TSH level might have resulted in a lack of correlation between MUIC and neonatal TSH levels.

The main strength of this study was the identification of recent tilt in the axis of iodine status in females, and the same finding even in pregnant mothers who were not exposed to povidone iodine. This emphasizes the fact that closer surveillance on both salt and non-salt sources of iodine should be done. Its long term implication has not been evaluated in terms of neonatal cognitive domain and general health. Extrapolation from adult data from India [10,11] suggests increasing autoimmunity and anti-TPO antibody positivity, which may have an impact on long-term neonatal health.

There were few limitations of this study. First, the collection of neonatal blood samples between 24-48 h may not be the best strategy for evaluating the TSH status. The proportion of population consuming iodized salt was recorded but we did not measure the content of iodine in salt samples on the basis of recall. Re-estimation of urinary iodine beyond the postpartum period or analysis of other thyroid parameters like thyroglobulin was also not performed in this study. As iodine is also excreted in breast milk, MUIC may lead to underestimation of iodine intake.

We conclude that neonatal TSH levels have no direct relationship with maternal urinary iodine levels; iodine insufficiency as well as excess seem to be higher among mothers of newborns with high (>5 mIU/L) TSH levels at 24-48 hours. Though iodine deficiency was still present in a proportion of the study samples, the looming burden of the excess is indeed a matter of concern. Few countries [12,13] have already entered the post-iodization era and report iodine excess. Further large scale multicentric studies across geographical zones, including coastal and mainland areas, are required along with robust parameters of assessment such as thyroglobulin and iodine content of the salt, relationship between maternal iodine and neonatal thyroid status.

Contributors: HS, SK, BKT: conceived the idea of the project; SY,RG: were involved in management; AJ, RB: was responsible for the laboratory an alysis; SG: managed the mothers antenatally; All the authors have contributed to manuscript writing and approved the final version.

Funding: Science and Engineering Research Board, 　New Delhi for NBS study in Delhi state vide Grant # IR/SO/LC-0001/2012.

Competing interest: None stated.

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