Vitamin d Dla

Question 1

What is vitamin D(calciferol)?

Answer 1

A group of sterols that have hormone like function

Question 2

Where can vitamin D be found?

Answer 2

Ergocalciferol(vitamin D 2)-found in plants

Cholecalciferol(vitamin D3)- found in animal sources

Question 3

What is the precursor to vitamin D(cholecalciferol) synthesis ?

Answer 3

The precursor for cholecalciferol (vitamin D3) synthesis in the skin is 7-dehydrocholesterol (intermediate in cholesterol synthesis )

Adequate exposure to sunlight can prevent vitamin D deficiency

Question 4

What are the factors affecting formation of vitamin D in skin?

Answer 4

• increased melanin pigment reduces its formation
• time to exposure and latitude
• Amount of exposed skin/ sunscreen reduces formation
• winter months and latitude

Question 5

What is the action of 24-hydroxylase?

Answer 5

Inactivation of 1,25-DHC
1,25-Dihydrixycholecalcoferol (calcitriol)

—> 1,24,25-hydroxycholecalciferol (inactive)

Question 6

What is the mechanism of action of 1,25 dihydrixycholecalciferol (calcitriol)?

Answer 6

• 1,25-dihydroxycholecalciferol(calcitriol) binds to intracellular receptor proteins in target cells(intestinal mucosal cells)
• 1,25-DHCC receptor complex interacts with DNA (hormone response element) in the nucleus of target cells (intestine)
• can either selectively stimulate gene expression or repress gene expression - increases synthesis of calbindins (similar mechanism of action of steroid hormones, thyroid hormones, retinoic acid)

Question 7

Contrast the action of vagina in D: serum calcium in the liver, kidney and intestine

Answer 7

On the intestine- stimulates intestinal absorption of calcium and phosphate by increased synthesis of a specific calcium binding protein (calbindin)

• on the bone- stimulates the mobilization of calcium and phosphate from the bone in the presence of parathormone (when the serum calcium is low)
• On the kidneys- inhibits calcium excretion by stimulating parathyroid dependent calcium reabsorption

Question 8

How does vitamin D affect serum phosphate?

Answer 8

Vitamin D increases serum phosphate by increasing its absorption from the intestine

• In the presence of vitamin D, serum calcium and phosphate are elevated - Increases the tendency to form bone mineral (Vitamin D facilitates mineralization, when serum calcium and phosphate are adequate)
• In vitamin D deficiency, reduced absorption of calcium reduces serum calcium level- stimulates secretion of PTH, which in turn causes demineralization of bone. Remember, PTH also increases excretion of phosphate in kidney
• Patients with vitamin D deficiency have low serum calcium and low serum phosphate levels and decreased bone mineralization

Question 9

What are the risk factors for vitamin D deficiency?

Answer 9

• inadequate exposure to sunlight(common in winter and elderly)
• nutritional deficiency-decreased intake/fat malabsorption/ exclusively breast fed infants
• chronic renal disease, chronic liver disease hydroxylation of vitamin D

Question 10

What are rickets?

Answer 10

Vitamin d deficiency in children

• decreased calcium absorption from the diet —> decreased serum calcium —> increased parathyroid hormone release—> increased demineralization of bone
• demineralization of bone - soft and pliable bones
• characteristic bow-leg deformity
• overgrowth at costochondral junction- rachitic rosary
• pigeon chest deformity
• frontal bossing, delayed closure of fontanelle
• delayed teething in infants

Question 11

What are the radiologic and lab findings of rickets?

Answer 11

Lab findings:

• elevated serum alkaline phosphate (ALP)
• Low serum calcium levels
• low serum phosphate levels

A) risckets- anteroposterior radiograph of wrist and hand in a 3

Question 12

What is osteomalacia?

Answer 12

Vitamin D deficiency as an adult

Bones are de-mineralized and are susceptible to fracture; may present as non-specific bone pain

• Osteomalacia can be secondary to reduced exposure to sunlight, dietary deficiency, renal disease or liver disease
• sub clinical vitamin D deficiency is common in the population (teens, young adults vitamin D deficiency affects the attainment peak of bone mass)

Question 13

What are the lab findings of osteomalacia?

Answer 13

• serum alkaline phosphatase activity is elevated

- serum calcium and phosphate levels are low

Question 14

Describe mutations in calcitriol receptor

Answer 14

• Inherited condition where calcitriol is unable to bind to its receptor in the intestinal mucosal cells (receptor mutation results in reduced binding of calcitriol)
• As a result, these children have high levels of calcitriol(loss of feedback inhibition)
• Serum calcium and phosphate levels are low but 1,25-hydroxyvitamin D levels are high
• compare vitamin D deficient rickets, which has low serum calcium and phosphate and low levels of 25-hydro XY vitamin D

Question 15

What is hypervitaminosis D?

Answer 15

Hypervitaminosis D is characterized by hypercalcemia (high serum calcium levels)- due to increased calcium absorption from the diet.

Vitamin D also increases phosphate absorption

• increase in serum calcium and phosphate results in an increased tendency for ectopic (soft tissues) mineralization
• Prolonged hypervitaminosis D (prescription medications can lead to calcification in soft tissues like kidney)

Question 16

Describe for,action of calcitriol (active form of vitamin D)

Answer 16

Cholecalciferol obtained from diet or skin by reaction: 7-dehydrocholesterol—> cholecalciferol

In the liver: cholecalciferol —> 25, hydroxycholecalciferol by 25-hydrolase

In the kidney:
25-hydroxycholecalciferol—> 1,25-dihydrixycholecalciferol(calcitriol)(1,25-DHCC) by 1-hydrolase

1-hydrolase stimulated by parathormone, low plasma calcium