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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