TL;DR

  • A baby depends entirely on the mother’s thyroid hormones early in pregnancy

  • Untreated thyroid deficiency has been linked to lower child IQ

  • Both low and high thyroid hormone levels may affect brain development

  • Iodine and selenium are critical, but balance matters

  • Testing is safer than guessing

Healthy thyroid function during pregnancy is critical for a baby’s brain development.

As early as 1999, research published in the New England Journal of Medicine showed that when a mother has untreated hypothyroidism during pregnancy, her child’s IQ may be affected. Children born to mothers with undiagnosed thyroid deficiency scored lower on IQ tests than children whose mothers had normal thyroid function. In this study, nearly 1 in 5 children born to mothers with untreated hypothyroidism had IQ scores of 85 or lower, a range linked with learning difficulties in school. On average, children ages 7–9 scored about 7 points lower on IQ testing when their mothers’ thyroid levels were low during pregnancy [1].

One reason this matters so much is timing. During the first 12 weeks of pregnancy, the developing baby relies entirely on the mother for thyroid hormones. During this early window, thyroid hormones play a key role in brain growth and nervous system development [1].

What Newer Research Shows

More recent studies have confirmed and expanded on these findings. A large meta-analysis published in 2018 found that thyroid hormone levels in early pregnancy were linked not only to child IQ, but also to autistic traits [2].

Another well-designed population study found that both low and high thyroid hormone levels during early pregnancy were associated with lower child IQ and changes in brain structure, including reduced gray matter volume. This research raised an important caution: pushing thyroid hormone levels too high with medication during pregnancy may also carry risks for brain development [3].

Thyroid Hormones, Iodine, and Child Development

Thyroid hormones depend on iodine, a mineral required to make thyroxine (T4). Several studies have examined iodine levels during pregnancy and early development.

In one study, researchers measured free thyroxine levels during the first trimester and followed children for two years. Children whose mothers had thyroid hormone levels in the lowest 25% showed more delays in motor development at 18 and 24 months than children whose mothers had higher levels [4].

Larger reviews and meta-analyses have since confirmed that adequate iodine intake during pregnancy is associated with better IQ outcomes in children [5]. Iodine is essential for brain development, and correcting iodine deficiency has been shown to improve cognitive performance in children [6]. However, excessive iodine intake can also disrupt thyroid function, highlighting the importance of balance rather than supplementation without testing. Click here to learn more about the importance of iodine.

Selenium Also Matters

Selenium is another nutrient required for normal thyroid hormone metabolism. Selenium deficiency has long been associated with cognitive decline in adults. Research now shows it also plays a role in childhood brain development.

Studies have found that low selenium status during pregnancy is associated with poorer cognitive outcomes in children during early childhood [7,8]. These findings suggest that thyroid health is influenced by multiple nutrients, not just hormones alone.

Bottom Line

Thyroid function during pregnancy plays a major role in a child’s IQ and brain development. Both low and excessively high thyroid hormone levels may affect neurodevelopment. Key nutrients involved include iodine and selenium, but more is not always better. Testing thyroid function and nutrient status is important, since simply boosting thyroid hormones or taking high doses of iodine may work against normal development.

References

  1. New England Journal of Medicine (1999;341:549-555, 601-602) MATERNAL THYROID DEFICIENCY DURING PREGNANCY AND SUBSEQUENT NEUROPSYCHOLOGICAL DEVELOPMENT OF THE CHILD
  2. J Clin Endocrinol Metab. 2018 Aug 1;103(8):2967-2979. Thyroid Function in Early Pregnancy, Child IQ, and Autistic Traits: A Meta-Analysis of Individual Participant Data
  3. Lancet Diabetes Endocrinol. 2016 Jan;4(1):35-43. Association of maternal thyroid function during early pregnancy with offspring IQ and brain morphology in childhood: a population-based prospective cohort study
  4. Journal of Pediatrics (2011 Sep;159(3):447-53) Association of maternal thyroid function during early pregnancy with offspring IQ and brain morphology in childhood: a population-based prospective cohort study
  5. J Clin Endocrinol Metab. 2019 Mar 28;104(12):5957–5967 Association of Maternal Iodine Status With Child IQ: A Meta-Analysis of Individual Participant Data
  6. Biology (Basel). 2022 Oct 14;11(10):1507. Iodine Intake and Related Cognitive Function Impairments in Elementary Schoolchildren
  7. Selenium status in pregnancy influences children’s cognitive function at 1.5 years of age Clinical Nutrition 34(5) October 2014
  8. Environ Health Perspect. 2017 Nov 7;125(11):117003. Early-Life Selenium Status and Cognitive Function at 5 and 10 Years of Age in Bangladeshi Children