Iodine is an essential trace element for the synthesis of thyroid hormones, which have been shown to be absolutely necessary for the development of the fetal brain during intrauterine life, starting from the second half of gestation. Most studies show that iodine deficiency and maternal-fetal hypothyroxinemia have negative effects on fetal neural maturation, dendritic arborization and synaptic formation. They delay the myelinization process and gliogenesis, which start in the second half of gestation and continue in postnatal life. Altered levels of iodine are correlated with defective brain development and neuronal maturation. Various degrees of irreversible neurocognitive defects that are caused by severe iodine deficiency, and subsequent maternal and fetal hypothyroxinemia are well-known. Recent studies further showed that, even in cases without clinical hypothyroidism, maternal hypothyroxinemia due to mild-to-moderate iodine deficiency would lead to fetal brain damage that could be reversed with early thyroxine therapy. Magnetic resonance spectroscopy (MRS) is a sensitive technique that detects alterations in brain metabolite levels in various neurodevelopmental disorders. Of these metabolites, N-acetyl aspartate (NAA) is exclusively present in neurons and their axons, and is therefore used to predict neuronal function and myelin maturation. Cerebral NAA level has been shown to decrease in almost all neuronal diseases, and is therefore extensively used in the evaluation of various demyelinizating and degenerative brain disorders. Choline (Cho) is the next most important cerebral metabolite, which reflects the rate of membrane turnover. Of many cerebral metabolites, only creatine (Cr) appears to be constant under various metabolic conditions; therefore, it is used as a reference value in semiquantitative assessments (e.g., a decreased NAA/Cr ratio is interpreted as a decline in NAA level). In untreated hypothyroidism, NAA/Cr ratio decreases, whereas Cho/Cr ratio increases. These ratios normalize with appropriate thyroxine therapy. Recently, intrauterine hypothyroxinemia due to iodine deficiency was also shown to cause a significant decrease in NAA/Cr ratio in neonates with hypothyroidism. Early thyroxine therapy caused normalization of NAA levels in these neonates.