Tuesday, December 11, 2012

ACTION OF THYROID HORMONES


ACTION OF THYROID HORMONES

This fraction of free hormone (fT3) is the active and can cross plasma membrane (due to low size) and directly bind to nuclear receptor (TR-RXR heterodimer) which are already bound to Thyroid hormone response cis-element in DNA (TRE) along with repressor, in the promoter region of target gene. Binding of hormone to receptor cause the release of repressor and subsequent recruitment of activators which will activate gene transcription. 

TR have high affinity and high specificity for T3. T3 binds to TR in target tissue with 10 times the affinity of T4. T3 and glucocorticoids enhance transcription of GH gene and so more GH is produced, the anabolic effect of T3 may be due to GH.
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