Vitamin D: Synthesis and Action

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Questions and Answers

What is the primary action of calcitonin in the body?

Increase bone resorption

Stimulate calcitriol synthesis

Inhibit osteoclast activity (correct)

Promote calcium absorption in the intestine

Which channel is involved in the absorption of calcium in the intestine as influenced by Vitamin D?

TRPV6 (correct)

NCX1

ClC-1

KCa3.1

In which organ does Vitamin D have a minor effect on calcium and phosphate reabsorption?

Pancreas

Kidney (correct)

Lung

Liver

What triggers the secretion of calcitonin?

Increased plasma Ca2+ levels Signup and view all the answers

Vitamin D affects plasma levels of which two substances?

Calcium and phosphate Signup and view all the answers

What role does vitamin D play in bone health?

Acts with PTH to increase bone resorption Signup and view all the answers

What is the half-life of calcitonin in the body?

10 minutes Signup and view all the answers

What is the major site of action for Vitamin D?

Intestine Signup and view all the answers

What condition is characterized by low levels of parathyroid hormone (PTH) and can result in hypocalcemia and hyperphosphatemia?

Hypoparathyroidism Signup and view all the answers

Which vitamin deficiency is primarily associated with rickets in children?

Vitamin D Signup and view all the answers

What is the most common cause of hypercalcemia in hospitalized patients?

Malignancy associated hypercalcemia Signup and view all the answers

Which treatment is recommended for primary hyperparathyroidism?

Surgical removal of parathyroid glands Signup and view all the answers

What condition results from the inability to produce 1,25 hydroxycholecalciferol due to chronic renal failure?

Vitamin D resistance Signup and view all the answers

Which condition is characterized by bending and softening of weight-bearing bones in adults?

Osteomalacia Signup and view all the answers

What type of hyperparathyroidism is caused by vitamin D deficiency or chronic renal failure?

Secondary hyperparathyroidism Signup and view all the answers

Which of the following is a characteristic feature of pseudohypoparathyroidism?

Defective PTH receptor Signup and view all the answers

Which hormone is responsible for increasing the re-absorption of bone?

Cortisol Signup and view all the answers

What is the serum calcium concentration that defines hypocalcemia in a patient with normal plasma protein concentration?

< 8.8 mg/dL Signup and view all the answers

What sign is characterized by carpopedal spasms following inflation of a sphygmomanometer cuff?

Trousseau's sign Signup and view all the answers

Which local factor is known to greatly increase osteoblast proliferation?

IGF 1 Signup and view all the answers

What is a common symptom of hypocalcemia related to sensory nerves?

Hyperexcitability Signup and view all the answers

Which hormone is necessary for fetal bone development?

Thyroxine Signup and view all the answers

What is the impact of thyroxine in adults regarding bone turnover?

Both anabolic and catabolic effects Signup and view all the answers

What does hypocalcemia primarily lead to in terms of muscle response?

Spontaneous twitching Signup and view all the answers

Study Notes

Vitamin D

Vitamin D, specifically 1,25-dihydroxycholecalciferol, increases both plasma calcium (Ca²⁺) and phosphate levels.
Vitamin D synthesis depends on calcium status, prolactin, parathyroid hormone, and oestrogens.

Vitamin D Synthesis Pathway

Vitamin D, is produced in the skin from 7-dehydrocholesterol via UV light.
Vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol) forms are ingested from diet/supplements.
The liver converts vitamin D3/D2 into calcidiol (25-hydroxyvitamin D).
The kidney converts calcidiol into calcitriol (1,25-dihydroxyvitamin D), the active form.
An inactive metabolite (24,25-dihydroxyvitamin D) is also produced in the pathway.

Vitamin D Action

The primary site of action is the intestines.
Vitamin D increases calcium and phosphate absorption (indirectly).
A calcium channel (TRPV6) in the intestinal lumen is involved.
Vitamin D induces the synthesis of calbindin D-28K, which binds and transports calcium across the intestinal cells.

Vitamin D Action (Kidney)

Reabsorbs calcium and phosphate in the kidneys.
This is a minor effect comparatively to other tissues.

Vitamin D Action (Bone)

Minor effect, but acts with PTH to increase osteoclast activity, accelerating bone resorption (old bone).
Stimulates new bone (new) mineralization due to elevated calcium levels.

Calcitonin

A hormone produced by parafollicular C cells of the thyroid.
Its structure consists of a peptide chain with a disulfide bridge.
Its molecular weight in humans contains 32 amino acids.
Acts quickly as its half-life is 10 minutes, but has minimal long-term effects.
Primarily inhibits osteoclast activity (bone resorption) when calcium plasma levels are high.
Decreases calcium and phosphate reabsorption in the kidneys.
Has no known pathophysiological deficiency consequences.
Important during infancy, pregnancy, and lactation.

Other Hormones Affecting Bone Turnover

Oestrogen and androgens decrease bone reabsorption.
Cortisol increases bone reabsorption.
Thyroxine is crucial for fetal bone development and stimulates peak bone mass accrual during growth.
Catabolic effects in adults can increase bone turnover.
IGF-1 increases osteoblast proliferation.
TGF increases osteoblast activity.
PGs increase bone turnover.
BMP stimulates bone formation.

Hypocalcaemia

Total serum calcium < 8.8 mg/dL (<2.20 mmol/L)
Serum ionized calcium < 4.7 mg/dL (<1.17 mmol/L)
Symptoms include hyperreflexia, twitching, muscle cramps, reduced threshold potential, tingling, numbness, muscle twitching, Trousseau, and Chvostek signs.

Hypocalcaemia Causes

Vitamin D deficiency (Dietary, absorption, metabolism issues)
Resistance to vitamin D.
Kidney disease (impeding the conversion of Vitamin D)

Hypoparathyroidism

Low parathyroid hormone (PTH) leads to hypocalcemia and hyperphosphatemia.
Possible causes include post-thyroid/parathyroid surgery, radioactive iodine (I₂) ablation, autoimmune/congenital causes.
Treated with hormone replacement therapy (oral calcium and vitamin D).

Pseudohypoparathyroidism

An inherited autosomal dominant disorder that results from a defective parathyroid hormone (PTH) receptor.

Hypercalcemia

Causes can be PTH-mediated (primary and tertiary hyperparathyroidism, familial hypocalciuric hypercalcemia, ectopic PTH secretion) or Vitamin D-mediated (excess ingestion, calcitriol excess) or non-PTH/non-Vitamin D mediated (PTH-related protein [PTHrP]-secreting tumor, calcitriol-secreting/stimulating tumor, bone metastases).

Primary Hyperparathyroidism

High calcium levels, increased bone resorption, calcium reabsorption in kidneys, and increased intestinal calcium absorption.
Results in high calcium excretion in urine.
Treatment includes surgical removal of the parathyroid glands.

Secondary Hyperparathyroidism

Caused by hypocalcaemia from Vitamin D deficiency or chronic renal failure.

Malignancy-Associated Hypercalcemia

The most common cause of elevated serum calcium in hospitalized patients.
It can be a result of PTHrP or various other mechanisms.

Osteoporosis

A decrease in total bone mass; due to dietary deficiency of calcium or vitamin D or vitamin C (for collagen synthesis) or related hormonal factors like menopause, immobilization, stress or drugs like anticonvulsants, glucocorticoids.
Characterized by decreased bone density and increased bone fragility.

Osteopenia

Decreased bone density.

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Description

Explore the essential roles of Vitamin D in regulating calcium and phosphate levels in the body. This quiz covers the synthesis pathway of Vitamin D and its mechanisms of action in the intestines. Test your knowledge on the various forms and functions of this vital nutrient.