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Questions and Answers
Which condition in children is characterized by insufficient calcium and phosphate for bone mineralization?
Which condition in children is characterized by insufficient calcium and phosphate for bone mineralization?
What is a primary cause of secondary hyperparathyroidism?
What is a primary cause of secondary hyperparathyroidism?
What is the treatment option for hypoparathyroidism?
What is the treatment option for hypoparathyroidism?
What role does 1,25-Dihydroxycholecalciferol play in the absorption of calcium?
What role does 1,25-Dihydroxycholecalciferol play in the absorption of calcium?
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Which of the following is a consequence of primary hyperparathyroidism?
Which of the following is a consequence of primary hyperparathyroidism?
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Which channel in the intestinal lumen is primarily responsible for calcium transport facilitated by Vitamin D?
Which channel in the intestinal lumen is primarily responsible for calcium transport facilitated by Vitamin D?
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Which hormone is involved in the regulation of calcium and phosphate levels in the body?
Which hormone is involved in the regulation of calcium and phosphate levels in the body?
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Inability to produce 1,25 hydroxycholecalciferol due to kidney issues leads to which condition?
Inability to produce 1,25 hydroxycholecalciferol due to kidney issues leads to which condition?
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What is the primary action of calcitonin in the body?
What is the primary action of calcitonin in the body?
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Which hormone is associated with increased bone re-absorption?
Which hormone is associated with increased bone re-absorption?
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Pseudohypoparathyroidism is caused by a defect in which specific biological structure?
Pseudohypoparathyroidism is caused by a defect in which specific biological structure?
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How does Vitamin D affect mineralization of new bone?
How does Vitamin D affect mineralization of new bone?
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Hypercalcemia of malignancy is primarily associated with which physiological change?
Hypercalcemia of malignancy is primarily associated with which physiological change?
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What triggers the secretion of calcitonin?
What triggers the secretion of calcitonin?
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What is the serum ionized Calcium concentration indicative of hypocalcemia?
What is the serum ionized Calcium concentration indicative of hypocalcemia?
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What is a feature of hypoparathyroidism?
What is a feature of hypoparathyroidism?
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What is the duration of action for calcitonin due to its half-life?
What is the duration of action for calcitonin due to its half-life?
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The presence of Trousseau's sign indicates what physiological condition?
The presence of Trousseau's sign indicates what physiological condition?
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Which of the following relates to the complications of hyperparathyroidism?
Which of the following relates to the complications of hyperparathyroidism?
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What effect does Thyroxine have on bone during early development?
What effect does Thyroxine have on bone during early development?
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Which hormone works alongside Vitamin D to enhance osteoclast activity?
Which hormone works alongside Vitamin D to enhance osteoclast activity?
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What is the primary site of action for Vitamin D in the body?
What is the primary site of action for Vitamin D in the body?
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Which local factor is known to increase osteoblast proliferation?
Which local factor is known to increase osteoblast proliferation?
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What indirect effect does Vitamin D have on calcium absorption?
What indirect effect does Vitamin D have on calcium absorption?
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What symptom is NOT commonly associated with hypocalcemia?
What symptom is NOT commonly associated with hypocalcemia?
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What consequence is suggested with calcitonin deficiency?
What consequence is suggested with calcitonin deficiency?
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What physiological mechanism leads to Trousseau's sign?
What physiological mechanism leads to Trousseau's sign?
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What role does BMP play concerning bone health?
What role does BMP play concerning bone health?
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What is the correct definition of total serum Calcium indicating hypocalcemia?
What is the correct definition of total serum Calcium indicating hypocalcemia?
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Which hormone has a catabolic effect on bone turnover in adults?
Which hormone has a catabolic effect on bone turnover in adults?
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Study Notes
Vitamin D
- Vitamin D (1,25-dihydroxycholecalciferol) increases plasma calcium and phosphate levels.
- Synthesis of Vitamin D depends on calcium status, prolactin, parathyroid hormone, and estrogens.
Vitamin D Synthesis
- Vitamin D3 (cholecalciferol) is synthesised in the skin from 7-dehydrocholesterol via UV light.
- Vitamin D2 (ergocalciferol) is obtained through diet or supplements.
- The liver converts vitamin D to calcidiol (25-hydroxyvitamin D).
- The kidney converts calcidiol to calcitriol (1,25-dihydroxyvitamin D), the active form of vitamin D.
- An inactive metabolite (24,25-dihydroxyvitamin D) is also produced in the kidney.
Vitamin D Action
- Intestine: Vitamin D increases calcium and phosphate absorption (indirectly). It does this by inducing the synthesis of calbindin D-28K, a protein that binds calcium, which then gets transported across the intestinal cell. The calcium channel in the gut lumen is TRPV6.
- Kidney: Vitamin D has a minor effect on calcium and phosphate reabsorption.
- Bone: Vitamin D has a minor role in bone resorption (in conjunction with PTH, on old bone), and in new bone mineralisation, which is driven by high calcium levels.
Calcitonin
- Calcitonin is a product of parafollicular C cells in the thyroid.
- It's a 32 amino acid peptide with a specific structure.
- Calcitonin's action is primarily short-term (T1/2 = 10 minutes), influencing calcium homeostasis.
- It lowers plasma calcium levels by inhibiting osteoclast activity in bone, preventing calcium release into the blood, and reducing calcium and phosphate reabsorption in the kidneys.
- There are no major pathological consequences from a lack of calcitonin. It might play a role in infancy, pregnancy, and lactation.
Other Hormones Affecting Bone Turnover
- Estrogen and androgens decrease bone reabsorption.
- Cortisol increases bone reabsorption.
- Thyroxine is crucial for fetal bone development and peak bone mass accrual during growth. In adults, it has a catabolic effect on bone turnover.
- Local factors such as IGF-1, TGFs (transforming growth factors), PGs (prostaglandins) and BMPs (bone morphogenetic proteins) play specific roles in osteoblast proliferation, activity, and bone formation.
Hypocalcemia
- Hypocalcemia is a condition marked by low total serum calcium (<8.8 mg/dL or <2.20 mmol/L) and serum ionized calcium (<4.7 mg/dL or <1.17 mmol/L) if normal plasma protein levels.
- Symptoms include hyperreflexia, twitching, muscle cramps, tingling, numbness, muscle weakness, and, potentially, Trousseau's and Chvostek's signs.
- Trousseau's sign is characterized by carpopedal spasms that result from occluding brachial artery blood flow.
- Chvostek's sign is a twitching in the facial muscles induced by a tap on the facial nerve.
Vitamin D Deficiency
- Causes can include dietary lack, inability to absorb vitamin D, or issues with metabolizing inactive vitamin D (e.g., kidney or liver disorders).
- Childhood deficiency manifests as rickets, skeletal deformities, and growth failure.
- Adult deficiency results in osteomalacia (soft bones) and difficulty bearing weight.
Vitamin D Resistance
- Kidney's inability to produce 1,25-hydroxycholecalciferol.
- Congenital absence or impairment of 1α-hydroxylase or chronic kidney disease.
Hypoparathyroidism
- Post-thyroid/parathyroid surgery, radioactive iodine (I₂), or autoimmune deficiency can cause this.
- Low parathyroid hormone (PTH), hypocalcemia and hyperphosphatemia are characteristic.
- Treatment includes hormone replacement therapy and calcium/vitamin D supplements.
- Pseudohypoparathyroidism is an inherited condition where PTH receptors don't work properly.
Hypercalcemia
- Hypercalcemia is a condition with elevated blood calcium levels.
- PTH-mediated causes include primary and tertiary hyperparathyroidism, familial hypocalciuric hypercalcemia, and ectopic PTH secretion.
- Vitamin D-mediated causes include excessive vitamin D ingestion or calcitriol excess.
- Not PTH/Vitamin D-mediated causes include tumors that secrete PTH-related protein (PTHrP), and those that affect calcium metabolism by bone metastases and destruction.
Hyperparathyroidism
- Primary hyperparathyroidism results in hypercalcemia, increased bone resorption and absorption of calcium (gut and kidney). Kidney stones are common. Treatment surgically removing the parathyroid glands.
- Secondary hyperparathyroidism occurs due to hypocalcemia from vitamin D deficiency or chronic kidney failure.
Malignancy-Associated Hypercalcemia
- Cancer can cause excessive calcium release from bones thus raising calcium levels in blood.
- The mechanism involves parathyroid hormone-related protein (PTHrP) increasing bone resorption of calcium.
- The PTH receptor family includes PTH1 and PTH2 receptors, and PTH, PTHrP, and TIP39 are the ligands that bind to these receptors.
- This condition is a common cause of hypercalcemia in hospitalized patients.
Osteoporosis
- Osteoporosis is a decrease in total bone mass.
- Causes include long-term dietary deficiencies of calcium and vitamin D, vitamin C deficiency (needed for collagen synthesis), immobilization, and menopause's reduced estrogen levels.
- Certain medications, such as anticonvulsants and corticosteroids, also contribute.
Osteopenia
- Osteopenia is a term for low bone density, representing a decrease in bone mass.
- May predispose someone to osteoporosis.
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Description
This quiz delves into the intricate details of Vitamin D, including its synthesis, action in the body, and factors influencing its production. Explore how Vitamin D affects plasma calcium and phosphate levels and its absorption processes in the intestine and kidney. Test your understanding of this essential nutrient and its biological significance.
