Calcium & Bone with Management of Parathyroid Disorders

Question 1

What are the key hormones and organs involved in calcium and phosphate regulation?

Answer 1

Hormones: Parathyroid hormone (PTH), Vitamin D, Calcitonin.
Organs: Bone, Kidney, Intestine.

Question 2

What percentage of calcium is found in bones, and what is the normal plasma concentration?

Answer 2

Bone: 99% of total body calcium.
Plasma Concentration: 2.1-2.6 mM.

Question 3

What stimulates the secretion of PTH and what is its half-life?

Answer 3

Stimulated by: Low calcium levels (e.g., alkalosis) and high phosphate levels (e.g., late CKD).
Half-life: Approximately 3 minutes.

Question 4

What are the primary actions of PTH on the bone, kidney, and gut?

Answer 4

Bone: Stimulates bone resorption.
Kidney: Increases calcium reabsorption (DCT), decreases phosphate reabsorption (PCT), and increases Vitamin D production (PCT).
Gut: Indirectly increases calcium absorption via Vitamin D.

Question 5

How is Vitamin D activated in the body, and what are its main effects?

Answer 5

Activation:
Skin: UV light converts 7-dehydrocholesterol to Vitamin D3.
Liver: Converts Vitamin D3 to 25-hydroxycholecalciferol.
Kidney: Converts 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol.

Effects:
Bone: Promotes bone resorption in synergy with PTH.
Gut: Increases calcium and phosphate absorption.
Kidney: Increases calcium and phosphate reabsorption.

Question 6

What triggers calcitonin secretion and what are its effects?

Answer 6

Triggered by: High plasma calcium levels.
Effects:
Bone: Decreases bone resorption.
Kidney: Decreases calcium reabsorption, little effect on phosphate.

Question 7

What is the role of the calcium-sensing receptor (CaSR) in regulating PTH secretion?

Answer 7

Role: When calcium levels are low, CaSR is not activated, which releases the inhibition on PTH secretion.

Question 8

What are the three main forms of calcium in plasma?

Answer 8

Calcium in plasma exists as:
Bound to albumin (45-50%)
Ionized calcium (45-50%)
Complexed to citrate, phosphate, or bicarbonate (5-10%)​

Question 9

What hormone primarily regulates calcium levels and how does it act on the bone?

Answer 9

Parathyroid Hormone (PTH) regulates calcium levels by stimulating bone resorption, mobilizing calcium and phosphate from the bone​

Question 10

What role does FGF23 play in phosphate homeostasis?

Answer 10

FGF23 reduces phosphate reabsorption in the proximal convoluted tubule (PCT) and increases phosphate excretion. It acts independently of PTH and is involved in downregulating Vitamin D to control phosphate levels​

Question 11

What are the common causes of hypercalcemia?

Answer 11

Common causes include:
Primary hyperparathyroidism
Malignancies (e.g., Multiple Myeloma, Humoral Hypercalcemia of Malignancy)
Vitamin D intoxication
Thiazide diuretics​

Question 12

What are the effects of hypocalcemia on the body?

Answer 12

Hypocalcemia can lead to:
Neuromuscular symptoms (tetany, seizures)
Cardiovascular issues (heart failure, arrhythmias)
Chronic effects (brittle nails, poor dentition, dry skin)​

Question 13

How does hyperparathyroidism differ from hypercalcemia due to malignancy in terms of lab findings?

Answer 13

Hyperparathyroidism: Increased PTH, calcium levels < 3.5 mmol/L, gradual onset.
Malignancy: Low or undetectable PTH, calcium levels > 3.5 mmol/L, rapid onset​

Question 14

What is the significance of 1,25-dihydroxyvitamin D in calcium regulation?

Answer 14

1,25-dihydroxyvitamin D increases calcium absorption from the gut and works with PTH to increase plasma calcium levels by reducing renal calcium clearance​