Parathyroid hormone exerts multiple actions that increase
extracellular calcium concentration, thus compensating for
the decreased concentration that originally stimulated secre-
tion of this hormone (
It directly increases the resorption of bone by
osteoclasts, which causes calcium (and phosphate) to
move from bone into extracellular ﬂ
It directly stimulates the formation of another
hormone, called 1,25-dihydroxyvitamin D, which
then increases intestinal absorption of calcium (and
phosphate). Thus, the effect of parathyroid hormone on
the intestinal tract is indirect.
It directly increases calcium reabsorption in the
kidneys, thus decreasing urinary calcium excretion.
4. It directly
the reabsorption of phosphate in
the kidneys, thus raising its urinary excretion. This
keeps plasma phosphate from increasing at a time when
parathyroid hormone causes an increased release of
both calcium and phosphate from bone, and increased
1,25-dihydroxyvitamin D is increasing both calcium
and phosphate absorption in the intestine.
denotes a group of closely related com-
is formed by the action
of ultraviolet radiation (from sunlight, usually) on a cholesterol
derivative (7-dehydrocholesterol) in skin.
is derived from plants. Both can be found in vitamin
pills and enriched foods and are collectively called vitamin D.
Because of clothing and decreased outdoor living,
people are often dependent upon dietary vitamin D. For this
reason, it was originally classiﬁ
ed as a vitamin. Regardless of
source, vitamin D is metabolized by the addition of hydroxyl
groups, ﬁ rst in the liver by the enzyme 25-hydroxylase and
then in certain kidney cells by 1-hydroxylase (
The end result of these changes is
Release of calcium
Calcium absorption into blood
Plasma parathyroid hormone
Parathyroid hormone secretion
Restoration of plasma calcium levels toward normal
Mechanisms that allow parathyroid hormone to reverse a reduction
in plasma calcium concentration toward normal. See Figure
11–33 for a more complete description of 1,25-(OH)
dihydroxyvitamin D). Parathyroid hormone and 1,25-(OH)
also involved in the control of phosphate levels.
Plasma vitamin D
Dietary vitamin D
Absorption of calcium
(and phosphate) into blood
Metabolism of vitamin D to the active form, 1,25-(OH)
Sarcoidosis is a disease that affects a variety of organs (usually
the lungs). It is characterized by the development of nodules of
amed tissue known as granulomas. These granulomas can
express signiﬁ cant 1-hydroxylase activity that is not controlled by
parathyroid hormone. What will happen to plasma calcium and
parathyroid hormone levels under these circumstances?
Answer can be found at end of chapter.