PARATHYROID HORMONE PhySiology
The function of para-
thyroid hormone is to maintain extracellular fluid calcium concentra-
tion. The Hormone acts directly on bone and kidney and indirectly
on intestine through its effects on synthesis of 1,25~0H)2D to increase
serum calcium; in turn, parathyroid honnone production is closely
regulated by the concentration of serum ionized calcium. This feedback
system is one of the most important homeostatic mechanisms. Any
tendency toward hypocalcemia, as might be induced by calcium-
deficient diets, is counteracted by an increased rate of secretion of
parathyroid hormone. This in turn (1) acts to increase the rate of
dissolution of bone mineral, thereby increasing the flow of calcium
from bone into blood, (2) reduces the renal clearance of calcium,
returning more of the calcium filtered at the glomerulas into extra-
cellular fluid, and (3) increases the efficiency of calcium absorption
in the intestine.
The relative physiologic importance in minute-to-
minute calcium homeostasis of these three actoins of parathyroid
hormone, stimulation of calcium transport in bone, kidney, and
intestine, is not clear, but most evidence suggests that immediate
iontrol of blood calcium is due to effects of the hormone on bone
and, to a lesser extent, on renal calcium clearance. Maintenance of
calcium balance, on the other hand, is probably due to the effects of
hormone on l,25(0H)2D levels and hence on the efficiency of
intestinal calcium absorption. As much as 12 mmol (500 mg) calcium
is transferred between extracellular fluid and bone each day (a large
amount in relation to the total extracellular fluid calcium pool), and
parathyroid hormone has a major effect on this transfer. The actoin
of the hormone tends to preserve calcium concentration in blood
acutely at the cost of bone destruction and bone mineral release. The
actoin of parath~oid hormone on kidney to increase the Eeabsorption
of filtered calcium may also contribute to rapid regulation of blood
calcium concentration.
~arathyroid hormone has a dual actoin on bone, the calcium
replacement and the bone remodeling effects.