We previously described a modification of the whole genome PCR method which 
allowed us to characterize several genes whose expression is regulated by 
thyroid hormone in the mouse liver. Following this procedure, we now report the 
identification of the mitochondrial NADH dehydrogenase subunit 3 (ND3) gene as 
target of thyroid hormone. ND3 gene expression is regulated by thyroid hormone 
in rat brain and heart. Sequencing and electrophoretic mobility shift assays 
confirmed the presence of a thyroid hormone receptor (TR)/c-erbA specific 
binding site in the mitochondrial ND3 gene. Hypothyroidism decreases ND3 mRNA 
levels in several brain areas such as cortex and hippocampus during the early 
postnatal development. In line with the recent findings showing the presence of 
TR/c-erbA alpha and beta proteins inside the mitochondria, our results suggest 
the possibility of direct transcriptional regulation of mitochondrial genes by 
thyroid hormone.