Thyroid hormone and the development of the brain

Thyroid hormone is an epigenetic factor in brain maturation. Its role is to synchronize developmental events by modulating the expression of aiready transcribed genes in a time- and region-dependent fashion. The genes regulated by thyroid hormone do not belong to a single functional category. Some of them are likely to represent primary actions of the hormone exerted directly at the genomic level. Others may be secondary to a more general effect on cell differentiation. Most of the genes are only transiently affected by hypothy roidism and thyroid hormone treatment during a critical period Several questions remain unanswered, such as the role of specific thyroid hormone receptor isoforms and the molecular basis for the time window and the region-specific control of gene expression.