Lipid-Soluble Hormones

Protein-Bound and Free Circulating Hormones

The liver produces proteins that bind lipid-soluble hormones, e.g.:

cortisol-binding globulin
thyroid-binding globulin
sex hormone-binding globulin (SHBG)

Equilibrium

e lipid-soluble hormone circulating in plasma bound to protein is in equilibrium with a small amount of free hormone. It is the free form that is available to the tissues, and thus the free unbound form normally determines the plasma activity. It is the free form that also creates negative feedback. is equilibrium is shown in Figure

Role of the liver

If the liver changes its production and release of binding proteins, the circulating level of bound hormone will change. However, under most conditions the level of free hormone will remain constant.

Modulation

Liver dysfunction and androgens can decrease and estrogens can increase the circulating level of binding proteins. For example, a rise in circulating estrogen causes the release of more binding protein by the liver, which binds more free hormone. e transient decrease in free hormone reduces negative feedback to the hormone-secreting tissue. e increased secretion of free hormone quickly returns the plasma free hormone to normal.

- This explains why during pregnancy and other states with a rise in estrogen levels:
- Total plasma lipid-soluble hormone increases.
- Free plasma hormone remains constant at a normal level; thus, the individual does not show signs of hyperfunction.

Page updated

Google Sites

Report abuse