The role of iodine and thyroid function in pregnancy and child development

Abstract

The thyroid gland produces the hormones triiodothyronine (T3) and thyroxine (T4) under the influence of thyroid stimulating hormone (TSH), with an adequate supply of these thyroid hormones during pregnancy critical for optimal foetal neurodevelopment. Sufficient levels of iodine, a micronutrient of which fish is the richest dietary source, are essential for optimal thyroid function and thereby child development. The overall aim of this thesis was to support existing knowledge on the important role of iodine and thyroid function during pregnancy on the cognitive outcomes of offspring within the Seychelles Child Development Study (SCDS), a high fish-eating cohort.

A comprehensive systematic review conducted within this thesis identified associations between maternal urinary iodine assessment, dietary iodine intakes and neurodevelopmental outcomes in children. The majority of pregnant women were classified as mild-moderately iodine deficient based on urinary iodine assessment and/or dietary intakes, with subsequent offspring neurodevelopment implications identified. To investigate the role of maternal thyroid function in neurodevelopment within the SCDS using data from the Nutrition Cohort 2 (NC2), two separate studies were conducted. Firstly, associations between maternal thyroid function and infant cognitive outcomes at 20 months of age were investigated. The second study investigated associations between maternal thyroid function and the incidence of child autistic/ADHD behaviours at 7 years of age. These investigations found no overall consistent associations. However, findings from these studies did indicate that a delicate balance in fT3 is required, given that variations may impact upon offspring neurodevelopmental outcomes.

An investigation of the iodine status of Seychellois children and associations with neurodevelopment at 7 years were also reported, with the majority deemed iodine sufficient according to their urinary iodine concentrations (UIC). Further, as part of this study, no associations between UIC and child cognitive outcomes at 7 years were reported. An exploratory analysis into the role of urinary analytes which may be related to iodine status were also investigated, with a number of urinary analytes found to be related to UIC. In an attempt to describe potential genetic factors regulating maternal thyroid hormone function, analysis of common Single Nucleotide Polymorphisms (SNPs) in selenoproteins were assessed. This investigation found no major associations between maternal SNPs and thyroid function and child neurodevelopmental outcomes. However, findings suggest that mothers heterozygous for the deiodinase 2 SNP (DIO2), compared to the homozygous reference genotype, had lower TSH concentrations.

Overall, the findings of this thesis have a number of important public health implications, notably, the Seychellois mothers as being euthyroid and thereby having assumed optimal thyroid function in tandem with the overall sufficient iodine status of the majority of the Seychellois children. In this population neither maternal thyroid function nor child iodine status appeared to influence neurodevelopment. Nevertheless, deficiency and excess of iodine were still reported among children, and as such there is still a need for improved knowledge, awareness, and education of the importance of iodine during pregnancy as a critical micronutrient for optimal pregnant and foetal outcomes.

Date of Award	Jan 2022
Original language	English
Supervisor	Alison Yeates (Supervisor), Maria Mulhern (Supervisor) & Emeir Mc Sorley (Supervisor)