The foetus relies on placental transfer of maternal thyroid hormones, until the thyroid matures fully, at ˜36 weeks gestation. Studies in animals, and the cognitive impairment experienced by children born in areas of iodine deficiency or to mothers with hypothyroidism, highlight the importance of thyroid hormone in brain development. The impact of less severe thyroid dysfunction remained controversial until two large-scale trials investigated the effect on child IQ of thyroxine supplementation in mothers with suboptimal gestational thyroid function (SGTF). In the controlled antenatal thyroid screening (CATS) study, women whose FT4 was <2.5th percentile and/or TSH >97.5th percentile were treated with 150 μg thyroxine from ˜13 weeks gestation. No differences were found in child IQ aged 3 from treated/untreated mothers. Similar results were obtained in a later trial which analysed subclinical hypothyroidism and hypothyroxinemia separately; no benefit was observed, from maternal treatment started at ˜17 weeks gestation, on childrens IQ at age 5. The results prompted questions as to whether the children were tested too young, the mothers were treated too late and/or supplementation dose was too high following the reported bi-phasic effect of FT4 on cognition. Repeated cognitive assessments in the CATS children at age 9.5 years confirmed the original findings, found that IQ at ages 3 and 9 were strongly correlated and that the lack of treatment effect may be due to the similar proportion of IQ < 85 in children of women with normal-GTF and SGTF. Behaviour problems were also measured in the CATS children using 3 questionnaires. We found no association between SGTF and offspring attention deficit hyperactivity disorder (ADHD), autism spectrum disorder or behaviour questionnaire scores. However, children of over-treated mothers (FT4>97.5th percentile) displayed significantly more ADHD symptoms and behavioural difficulties than normal-GTF. Thus thyroxine supplementation during pregnancy requires careful monitoring to avoid over-treatment.
27 Nov 2019 - 29 Nov 2019