Introduction to Parathyroid Disorders and Calcium Metabolism

Question 1

What is the physiological function of calcium? [4]

Answer 1

1. transmission of nerve impulses
2. muscle contraction
3. regulating heartbeat
4. strong bones and teeth

Question 2

How is plasma calcium distributed? [3]

Answer 2

1. ~40% bound to albumin - unavailable to tissues
2. ~15% as complexed calcium
3. ~45% as freely ionised - metabolically active

Question 3

How is parathyroid hormone (PTH) secreted? [3]

Answer 3

PTH secreted by chief cells of the parathyroid gland / in response to low ionised Ca2+ levels / detected by specific calcium-sensing G-protein-coupled receptors on the plasma membrane of the parathyroid glands

Question 4

What are the effects and overall effect of PTH? [4]

Answer 4

1. overall effect of increased Ca2+ and decreased PO4(3)-
2. increased osteoclast activity and bone resorption so increased Ca2+ and PO4(3)-
3. increased Ca2+ and decreased PO4(3)- reabsorption in the kidney
4. increased renal production of 1,25-dihydroxyvitamin D

Question 5

How is the active form of vitamin D produced? [2]

Answer 5

vitamind D hydroxylated in the liver to 25-hydroxyvitamin D / then hydroxylated again in the kidney to 1,25-dihydroxyvitamin D (calcitriol)

Question 6

What is calcitriol production stimulated by? [3]

Answer 6

1. decreased Ca2+
2. decreased PO4(3)-
3. increased PTH

Question 7

What are the actions of calcitriol? [4]

Answer 7

1. increased Ca2+ and PO4(3)- absorption in the gut
2. decreased PTH release
3. enhanced bone turnover
4. increased Ca2+ and PO4(3)- reabsorption in the kidney

Question 8

Where is calcitonin made and what is its action? [2]

Answer 8

1. made in the C-cells (parafollicular cells) of the thyroid

2. causes decreased Ca2+ and PO4(3)- by inhibiting osteoclast activity

Question 9

What is the affect of magnesium on PTH? [2]

Answer 9

1. decreased Mg prevents PTH release

2. may cause hypocalcaemia