Mechanism of Thyroid Hormone

[Pantaloons]

'Allo ladies and gents:

 

I know that the Thyroid hormone a tyrosine derivative, and is therefore hydrophilic. Thus, TH should bind to a surface protein receptor and initiate a secondary cascade, no?

 

TPR, however, writes

"The mechanism of action of thyroid hormone is to bind to a receptor in the cytoplasm of cells that ten regulates transcription in the nucleus.

 

That sounds like what a hydrophobic hormone would do, not a hydrophilic one such as the tyrosine derivatives.

 

What am I missing here?

Thanks!

[doc_911]

There are two types of Tyrosine derivatives: water soluble and lipid -soluble.

 

Water -soluble are: catecholamines formed in adrenal medulla (epinephrine and norepinephrine), - these ones dissolve in blood and bind to a recptor, acting thru a 2 messenger cAMP system.

 

Lipid-soluble are: thyroid hormones T3 and T4 ( calcitonin is a peptide homone, not a Tyr-derivative and is water-soluble). These ones are carried by plasma proteins, then diffuse thru the cell membrane and go straight for the receptors inside the nucleus. Because they end up interacting with a bunch of proteins they meet on their way thru tyhe cytosol - the effect of these gormones is REALLY SLOW. Eventually they reach the nucleus and promote transcription. Basal Metabolism goes up in nearly every cell of the body due to the Tyr-derivatives T3 and T4 of the thyroid.

 

Hope it makes sense;)

[metukah]

The important thing to remember about TH is that it is a PEPTIDE, but it acts like a STEROID (ie. binds in the cytoplasm and alters transcription).

TH is the only exception.

The other hormones fit nicely into the classes of PEPTIDE or STEROID.

[Pantaloons]

Thank-you for the responses!

 

Best of luck!