Thyroid Hormones and Calcium Balance

Question 1

What is the general function of calcitonin?

Answer 1

Calcitonin decreases [Ca2+] of blood by stimulating osteoblast activity and inhibiting osteoclast activity.

Question 2

Describe the hormonal control of calcium metabolism between ECF and GIT

Answer 2

Diet (GIT) -> ECF via calcitrol (PTH, Prolactin)

ECF (secreted into s.intestine) -> Faeces

Question 3

Describe the hormonal control of calcium metabolism between the ECF and Bone

Answer 3

Bone -> PTH/calcitriol/cortisol -> ECF

ECF-> Calcitonin-> Bone

Question 4

Describe the hormonal control of calcium metabolism between the ECF and kidney.

Answer 4

ECF -> passive filtration -> Kidney

Kidney -> PTH(stimulates travel), calcitonin (inhibits travel)-> ECF

Question 5

Describe the hormonal control of calcium metabolism between the ECF and tissue cells

Answer 5

ECF -> Electrochemical gradient -> Cells

Cells -> Active Transport -> ECF

Question 6

What is the difference between calcium homeostasis and calcium balance?

Answer 6

Ca2+ Homeostasis = minute to minute basis, rapid exchange of Ca2+ between bone and ECF compartment, and to a lesser extend, modifications in urinary excretion of Ca2+.

Ca2+ balance = slow response, PTH is principal regulator. Acts directly or indirectly on bone, kidneys or gut. Calcitonin used during hypercalcaemia. Vit D contribution. Constant total Ca2+ maintanence, hormonal control

Question 7

Describe the actions of calcitonin.

Answer 7

Inhibits osteoclast activity and limits bone reabsorpion to lower blood [Ca2+].

Short Term: induces decreased Ca2+ efflux across osteocytic-osteoblastic membrane.

Long term: Induces decreased bone reabsorption by inhibiting osteoclast action and also promoting the stimulation of Ca2+ excretion at the kidneys.

Question 8

Descibe the actions of the parathyroid hormone.

Answer 8

Overall role of raising free plasma Ca2+

PTH mobilises Ca2+ stores to increase ECF concentration.

2 Major effects:

• Induces fast Ca2+ efflux into plasma from small labile pool of Ca2+ from bone fluid.
• Stimulates bone dissolution: promotes osteoclast activity. (Issue= bone dissolution may lead to increaseed [Po₄^3-]

At kidneys, PTH promotes Ca2+ conservatino (renal threshold increased, also excretes[Po₄^3-].

At intestines, PTH indirectly increaes Ca2+ and Po₄^3-absorption. Vit D increases gut absorption of Ca2+ and Po₄^3-.

Question 9

Describe the formation and actions of calcitriol.

Answer 9

Vit D = greater absorption of Ca2+

Vit D contributes to Ca2+ balance but not homeostasis.

In absense PTH maintains plasma [Ca2+] at the expense of bones (ricketts/ osteomalacia)

1. Diet/ UV absorption from sun-> body
2. cholecalciferol ->liver -> 25-hydroxycholecalciferol
3. 25-hydroxycholecalciferol -> kidneys -> calcitriol
4. Increased plasma Ca2+