Clinical_Rounds_in_Endocrinology_Volume_II_-_Pediatric_Endocrinology

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16. Why is vitamin D considered as a hormone?

Vitamin D is not truly a vitamin, but it is a steroid and qualifies the criteria

for a hormone. These include endogenous production (Malpighian layer of

epidermis), direct release into circulation [1,25(OH) 2 D release from kidney],

presence of binding protein (vitamin D-binding protein), action through a

receptor, existence of a feedback system [between 25(OH) vitamin D and

1,25(OH) 2 D], and site of action distal to organ of production (kidney, intes-

tine, and bone).

17. How is vitamin D important in bone physiology?

1,25(OH) 2 D, the active form of vitamin D, plays an important role in bone

health. It is the prime regulator of calcium and phosphorus absorption from

small intestine. 1,25(OH) 2 D by increasing the synthesis of osteocalcin promotes

maturation of mineralized matrix. In addition, it activates the differentiation of

osteoclast precursors and thereby promotes bone remodeling. Although vitamin

D is important for bone mineralization, it is not essential as evidenced by healing

of rachitic lesions in patients with inactivating mutations of vitamin D receptor

(vitamin D-resistant rickets type 2) after treatment with intravenous calcium.

Sun

UV B

7 - dehydrocholesterol Vitamin D 3 Skin (80% of total vitamin D)

25 (OH) vitamin D 2 /D 3

Kidney (1α - hydroxylation)

Liver

(25-hydroxylation)

Diet (20% of total vitamin D)

Vitamin D 2

Vitamin D 3

1, 25 (OH) 2 vitamin D 2 /D 3

Fig. 5.7 Vitamin D biosynthesis

5 Rickets–Osteomalacia