Parathyroid Gland : Calcium And Phosphate Regulation

Question 1

How many grams of calcium do adult humans contain ?

Answer 1

1000g

Question 2

Around how many mg of calcium is exchanged between bone and ECF each day ?

Answer 2

300-600mg

Question 3

What is normal serum concentration ?

Answer 3

2.2-2.6 mM

Question 4

In the ECF , what percentage of calcium is ionised , protein bound and complexed ?

Answer 4

47% is ionised ( active form )

47% is protein bound ( inactive form )

6% is complexed ( eg with Pi , citrate , carbonate )

Question 5

Where is most calcium stored ?

Answer 5

In the skeleton

Question 6

Why is calcium important ?

Answer 6

1. Builds and maintains bones and teeth
2. Regulates heart rhythm
3. Eases insomnia
4. Helps regulate the passage of nutrients in and out of cell walls
5. Assists in normal clotting of blood ( factor 4 )
6. Helps maintain proper nerve and muscle function
7. Lowers blood pressure
8. Reduced blood cholesterol levels
9. Reduces chances of colon cancer

Question 7

What is EDTA ?

Answer 7

When taking a blood sample , you add EDTA which prevents blood from clotting as it removes calcium by chelating it. This is because calcium is an important clotting factor ( factor 4). This helps to preserve the blood for longer

Question 8

Why do we need to giv IV calcium in blood transfusions?

Answer 8

Because when blood donors donate their blood - the blood is put into a bag which contains citrate. Citrate chelates calcium ions. And so when giving blood transfusions to patients - blood will have low calcium levels.

And so we need to give massive transfusions of IV calcium.

Question 9

What are the three hormones involved in regulation of calcium and phosphate ?

Answer 9

1. Parathyroid hormone
2. Calcitriol
3. Calcitonin

Question 10

Normally , how many parathyroid glands do we have ?

Answer 10

4

Question 11

What cells secrete the parathyroid hormone ?

Answer 11

Chief cells - stained darker than oxyphil cells

Question 12

Why is parathyroid hormone known as an acute hormone in calcium regulation ?

Answer 12

Because the effects it takes are very quick - it has Avery short half life of 4.5 minutes. And after that is cleaved by the liver.

Question 13

Does PTH move freely in the blood or is it bound to a protein ?

Answer 13

Has no serum binding protein

Question 14

What type of hormone is PTH ?

Answer 14

Polypeptide hormone

Question 15

What is the f7nction of oxyphil cells ?

Answer 15

No function known

Question 16

Synthesis of PTH is regulated both at transcriptional and post transcriptional levels , what occurs at low serum and high serum calcium levels ?

Answer 16

Low serum calcium up regulates gene transcription
High serum calcium down regulates gene transcription
Low serum calcium prolongs survival of mRNA.

Question 17

What are PTH target organs and physiological effects ?

Answer 17

1. PTH will bind to receptors on the bone which would increase the activity of osteoclasts and decreases activity of osetoblasts. which would degrade bone releasing calcium into the bloodstream
2. The intestine : PTH activates the inactive form of vitamin D into Calcitriol which is the active form of vitamin D. Vitamin D is required for the resorption of dietary calcium from GI tract.
3. the kidney : decreases loss of calcium to urine by increasing calcium reabsorption by the kidneys and and increases phosphate loss into the urine.

Question 18

Outline the process by which the activated form of vitamin D is synthesised ?

Answer 18

1. Vitamin D3 ( cholecalciferol) is synthesised by sunlight and cholesterol. This is hydroxylated in the liver into 25(OH)D
2. 25(OH)Dis the pre hormone substrate. It is produced in the liver from D3 ( cholecalciferol). In the kidneys , 25(OH)D is activated by the PTH and hydroxylated again.
3. 1,25( OH ) 2D also known as calcitriol is the activated version of vitamin D. It is produced in the kidney.

Question 19

What is the difference between D3 to hormones ?

Answer 19

Vitamin D38# not secreted by classical endocrine glands

Vitamin D3 is formed in the skin or from the diet.

Question 20

What are the effects of Calcitriol ( 1,25(OH)2D ?

Answer 20

1. Plays an essential role in the gut , it aids with the reabsorption of dietary calcium.
2. Bone : increases Ca2+ removal
3. Kidney : signals for calcium to be rebabsorbed by the kidney and stimulates phosphate excretion .

Question 21

Where is calcitonin secreted from and what is its main function ?

Answer 21

Secreted from the C cells in the thyroid gland ( lay around the thyroid follicle ).

Their main function is to decrease calcium serum levels by retaining calcium in the skeleton by preventing bone breakdown , increasing bone building, decreases ca2++ reabsorption by the gut , decreases calcitriol levels which all dcereases plasma Ca2+ Levels.

Question 22

What is hypercalcaemia? And what are associated consequences?

Answer 22

1. This is when serum calcium levels rise above 3 mmol/L

The high calcium levels leads to polyuria which leads to dehydration.

‘ STONES , MOANS , GROANS AND ACHES ‘

STONES = renal calculi ( renal stones ) 
GROANS = severe constipation which leads to abdominal pain 
MOANS = depression / tiredness / lethargy 
BONES = muscle aches

Question 23

Main treatment for hypercalcaemia?

Answer 23

Rehydration eases symptoms

Question 24

What are the two main causes of severe hypercalcaemia?

Answer 24

1. Malignant osetolytic bone metastasis which increases bone degradation due to increase activity of osetoclasts. The common cancers that metastasise to bone is : breast , lung , renal , thyroid cancers.
2. Squamous tumours in the head , neck and lungs. This tumour secretes parathyroid hormone receptor peptide. This acts on parathyroid hormone receptors which increases serum calcium levels.

Question 25

What are common sites for bone metastasis?

Answer 25

1. Vertebrae 
Pelvis 
Proximal parts of the femur 
Ribs 
Proximal parts of the humerus 
Skull,

Question 26

What are the two causes of hyperparathyroidism?

Answer 26

1. Primary cause : one of the 4 parathyroid glands develop an adenoma and secretes excessive parathyroid hormone. This causes calcium to rise and serum phosphate to fall.
2. Secondary causes : All 4 parathyroid glands become hyper plastic. This is seen in vitamin D deficiency patients. Vitamin D deficiency means that their calcium absorption is low resulting in low serum calcium levels which then causes PTH levels to rise.

Question 27

What are symptoms of hyperparathyroidism?

Answer 27

STONES = kidney stones , also polyuria due to impaired sodium and water rebaosptiom

2. MOANS = depressed , tired , exhausted
3. GROANS : constipation , peptic ulcers and pancreatitis
4. BONES= bone and muscle aches

Question 28

What is the difference between osteomalacia and osteoporosis?

Answer 28

1. Osteomalacia results in defective mineralistion of bones this is due to vitamin D deficiency which results in less calcium being absorbed by the gut etc. NO amount of dietary calcium can improve this. Only vitamin D supplements. This is the adult version of ‘ rickets ‘ in children. This leads to soft bones.
2. Whereas osteoporosis is due to old age etc - the bone is fully mineralised but it is structurally degraded