*2.4.1. General genomic action (Table 5 and Figure 2)*

T4 and T3 enter the cell through transporter proteins such as MCT8 and 10 or OATPs. Inside the cells, deiodinases (DI, II) convert T4, the major form of thyroid hormone in the blood, to the more active form T3. DIII produces rT3 and T2 from T4 and T3, respectively [1,73,83]. T3 binds to nuclear TRs, TRα and TRβ, that activate transcription by binding, generally as heterodimers with the retinoid X receptor (RXR) (Table 5) [87], to thyroid hormone response elements (TREs) located in regulatory regions of target genes [84]. Activity is regulated by an exchange of corepressor (CoR) and coactivator (CoA) complexes. Negative TREs (nTRE) can mediate ligand-dependent transcriptional repression, although in this case the role of coactivators and corepressors is not well defined [73,85]. TRs can also regulate the activity of genes that do not contain a TRE through "cross-talk" with other transcription factors (TF) that stimulate target gene expression [28,86]. Both receptors and coregulators are targets for phosphorylation (P) by signal transduction pathways stimulated by hormones and growth factors [84,85]. Thus, the nuclear actions of T3 are sensitive to inhibitors of transcription and translation and have a latency of hours to days [9,73]. Thus, the genomic action will play a critical role in the cellular proliferations and differentiations.
