Parathyroid glands

[jtaylor@pyramid.net]

There was a article in endurance news a awhile back about the
parathyroid glands.  The article stated that horses fed alfalfa excrete
huge amounts of calcium, and their parathyroid glands "go to sleep"
Therefore, train endurance horses on grass or oat hay, to keep the
parathyroid alert and active, and give alf just before and during
competition. My question is, what and where is the parathyroid how does
it work and how does a horse react if this thing is active or not?