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Chapter 31: Drug Therapy of Mineral Ion Homeostasis and Bone Turnover Disorders

A 43-year-old man with intestinal bypass surgery is at risk of developing a deficiency of

a. vitamin D.

b. calcium.

c. phosphorous.

d. vitamin C.

e. parathyroid hormone.

Answer is a. Bile is essential for the absorption of vitamin D. Patients who have intestinal bypass surgery or otherwise have severe shortening or inflammation of the small intestine may fail to absorb vitamin D sufficiently to maintain normal levels.

25-hydroxyvitamin D (25-OHD) is converted to its active form 1,25-OHD by the enzyme 1α-hydroxylase in the proximal renal tubules. 1α-hydroxylase is activated by a dietary deficiency of

a. potassium.

b. iron.

c. calcium.

d. zinc.

e. sodium.

Answer is c. See Figure 31-1. Vitamin D 1α-hydroxylase is subject to tight regulatory control that results in changes in calcitriol formation appropriate for optimal calcium homeostasis. Dietary deficiency of vitamin D, calcium, or phosphate enhances enzyme activity. Conversely high calcium, phosphate, or vitamin D intake suppresses enzyme activity.


Figure 31-1. Regulation of 1α-hydroxylase activity. Changes in the plasma levels of PTH, Ca2+, and phosphate modulate the hydroxylation of 25-OH vitamin D to the active form, 1,25-dihydroxyvitamin D. 25-OHD, 25-hydroxycholecalciferol; 1,25-(OH)2-D, calcitriol; PTH, parathyroid hormone.

Calcitonin is a hormone produced and secreted from the thyroid parafollicular C cells and is regulated by the serum concentration of

a. potassium.

b. calcium.

c. zinc.

d. iron.

e. sodium.

Answer is b. The biosynthesis and secretion of calcitonin are regulated by the serum calcium concentration. Calcitonin secretion increases when serum calcium is high and decreases when serum calcium is low.

A 42-year-old man with lung cancer had a serum calcium measurement 1 month ago that was 9.0 mg/dL. He is hospitalized now with severe muscle weakness, fatigue, anorexia, and constipation. His serum calcium is 13.5 mg/dL. The severity of his symptoms is due to the

a. rate of rise in his serum calcium.

b. decrease in the intestinal ...

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