STEROID HORMONE
These are fats similar to, and usually synthesized from,
cholesterol, starting with Acetyl-CoA, moving through squalene, past lanosterol, into
cholesterol, and, in the gonads and adrenal cortex, back to a number of steroid
hormones. Nearly all of the classic hormones are proteins or smaller peptides; they don
get inside a cell (the membrane keeps them out); instead, they bind to, and initiate, cell
changes from the outside. The exceptions are the thyroxines (from the thyroid) and the
steroid hormones. They move into the cell, bind with receptors, and initiate changes in
the way a cell regenerates itself or synthesizes new compounds. Because the steroid
hormones stimulate cell growth, either by changing the internal structure or increasing
the rate of proliferation, they are often called anabolic steroids. Estrogen, an ovarian
steroid, when secreted into the bloodstream, will be bound within a short time by internal
receptors inside those cells that need estrogen for their growth; the unused portion is
partially broken down, mostly in the liver, and partially stored in a less active form by
adipose tissue. Since luteinizing hormone from the pituitary is surged in pulses an hour
apart, the estrogen is also surged from the reacting ovaries, and by the time more
estrogen is available, the binding cells need more; their program of synthesis has run out
and needs to be started again. Of course, most steroid hormone reactions are less
measured than this, but you get the idea.