section 37.1
Calcium and Phosphorus
877
TABLE 37-1 (
c o n tin u e d
)
Factor
Pathological Effects on Calcium
and Phosphorous Metabolism*
Mechanism/Site of Action*
D iet
Inadequate C a2+ intake produces
rickets or osteom alacia;
hypophosphatem ia rarely (if ever)
results from dietary inadequacy;
ex cess dietary Ca2+ causes
hypercalciuria and risk o f
urolithiasis.
C a2+ and inorganic phosphate are
absorbed in the sm all bow el by
active l,2 5 -(O H )2D -requiring
processes; Ca2+ decreases PTH
secretion and m ay decrease la -
hydroxylase activity; inorganic
phosphate decreases la -
hydroxylase activity.
Estrogens
D ecreased estrogens are probably
responsible for postm enopausal
osteoporosis.
E strogens increase renal la -
hydroxylase activity. Prom otes
osteoblast differentiation.
G lucocorticoids*
O steoporosis occurs in C ushing’s
disease and in patients treated
w ith glucocorticoids for
im m unosuppression.
G lucocorticoids are used in the
treatm ent o f hypercalcem ia.
D ecrease intestinal Ca2+
absorption; m ay antagonize 1,25-
(O H )2D or PTH; m ay directly
stim ulate parathyroids; m ay have
direct effects on intestine and
bone independent o f PTH and
l,2 5 -(O H )2D.
Growth h orm one/insulin-like
grow th factors*
A bsence during bone growth causes
slow grow th and short stature
(dw arfism ).
U nknow n; m ay not be directly
related to calcium and vitam in D
m etabolism .
Insulin*
D ecreased bone m ass in insulin-
dependent diabetes; in rats,
decreased insulin low ers intestinal
Ca2+ absorption, intestinal CaBP,
and serum l,2 5 (O H )2D.
Probably increases conversion o f
25-(O H )D to l,2 5 -(O H )2D.
Prolactin*
N o pathological effects are known.
In chicks, prolactin adm inistration
increases renal la -h y d ro x y la se
activity and serum l,2 5 -(O H )2D
concentration.
Thyroid horm ones*
B on e abnorm alities are seen in
hyper- and hypothyroidism .
U nknow n; m ay affect vitam in D
m etabolism .
*Effects observed in humans unless otherwise noted.
^Results are from animal studies; some of these mechanisms may also explain effects observed in humans.
*These hormones have other major roles in the body, and their effects on calcium and phosphate homeostasis are generally of secondary importance.
