Genomic imprinting of the type 3 thyroid hormone deiodinase gene: Regulation and developmental implications

► The type 3 deiodinase inactivates thyroid hormones thus limiting their action. ► D3 plays important roles during development and in adult physiology. ► The D3 gene is imprinted and preferentially expressed from the paternal allele. ► Dlk1-Dio3 locus genomic elements and epigenetic marks direct Dio3 allelic expression. ► Altered imprinted gene dosage in the Dlk1-Dio3 cluster causes abnormal phenotypes.