Endocrine aspects of development. Thyroid hormone actions in neurological processes during brain development

Abstract Hormones are key factors throughout neurodevelopment, and thyroid hormones are essential for a correct maturation of the nervous system. Thyroid hormones’ pleiotropic actions during brain development include the regulation of cell generation and survival, differentiation, migration, growth, and retraction of cellular processes, synaptogenesis, myelination, and establishment of neuronal circuits. Indeed, thyroid hormones act on different progenitor cells, glial cells, and neurons subtypes, at specific regional and temporal patterns. They are key factors involved in the regulation of the expression of target genes, including cell cycle proteins, growth factors, and neurotrophins, cytoskeleton proteins, axon guidance and synaptogenesis-related proteins, myelin proteins, and enzymes related to neurotransmitter metabolism. In the fetus, the maternal thyroid hormone supply is essential for a correct neurodevelopment, until the fetal thyroid gland is functional. Thyroid hormones deficiency during brain development induces mild to devastating impairments on the neurological outcome of the offspring depending on the severity of the deficiency, its duration, and the time window affected.