Introduction: The focus of the present study was the importance of the mother’s thyroid function for foetal development in the first trimester, when the baby is totally dependent on the mother for thyroid hormones. Material and Methods: The study consisted in testing the intellectual performance of children with both euthyroid and thyroid-dysfunction mothers. The experimental group comprised 60 children of mothers with an untreated thyroid disorder in the first trimester of their pregnancy (TSH≥3.5 and/or TPO-Ab≥20). The control group contained 132 children whose mothers showed no symptoms of a thyroid disorder either before or during pregnancy/postpartum. Both groups of children were administered the WISC-III, whereby the intellectual performance of the experimental-group children was compared with that of the control-group children. The comparison included the percentage of children with IQ≤ 85 and SLD and/or ADD risks. Our research is a follow-up to a blanket thyroid screening of 1 649 pregnant women conducted during 2004-2006 in the region around Havlíčkův Brod. Results: The research found no significant difference between the two groups of children with respect to their intellectual abilities, either regarding their overall IQ (p=0.67), verbal IQ (p=0.81), performance IQ (p=0.41), or the individual scores (ISP: p=0.85; IPU: p=0.54, IKO: p=0.57; IRZ: p=0.13), nor did the experimental group show a significantly higher occurrence of children with IQ≤85 than the control group (p=0.66). However, the experimental group did exhibit a statistically significant increase in the percentage of children with a suspected SLD or clinically significant attention issues (p =0.05). Conclusion: Untreated thyroid disorders in the first trimester of pregnancy can increase the risk of the child developing attention or learning issues.

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