Parathyroid Gland Flashcards
List 3 types of functions of the skeleton.
1 - Mechanical.
2 - Protective.
3 - Metabolic.
List 4 substances involved in calcium homeostasis.
1 - Parathyroid hormone (PTH).
2 - Vitamin D.
3 - Calcitonin.
4 - Fibroblast growth factor 23 (FGF23).
List 3 diseases of bone.
1 - Hyperparathyroidism.
2 - Osteomalacia.
3 - Osteoporosis.
What proportion of serum calcium is free?
To which molecule is the rest of the calcium bound?
- 50%.
- The rest is bound to albumin.
How many parathyroid glands are there?
4.
List the factors that stimulate the parathyroid gland to secrete parathyroid hormone.
1 - Low calcium.
2 - High phosphate.
List 4 actions of parathyroid hormone.
1 - Increases calcium reabsorption and decreases phosphate reabsorption in the renal distal tubule.
2 - Increases both calcium and phosphate reabsorption in the intestine via activation of vitamin D.
3 - Increases calcium release from bone via activation of osteoclasts.
4 - Decreases phosphate reabsorption.
How many amino acids comprise the parathyroid hormone?
Which of these amino acids are responsible for its biological activity?
- 84 amino acids in total.
- The first 34 amino acids are responsible for its biological activity.
What is the normal adult reference range for parathyroid hormone?
1.6-6.9 pmol/L.
To which receptors does parathyroid hormone bind?
Where in the body are these receptors found?
Parathyroid hormone binds to GPCRs mainly in the kidney and osteoblasts.
Describe the negative feedback mechanism for parathyroid hormone.
- PTH transcription is inhibited by 1,25-(OH)2 D3.
- PTH transcription is also inhibited by increased serum calcium.
What is the difference between vitamin D2 and vitamin D3?
Vitamin D2 is of plant origin whereas vitamin D3 is of animal origin.
Are vitamin D receptors extracellular or intracellular?
Intracellular (vitamin D is a steroid).
List 2 dietary sources of vitamin D
1 - Eggs.
2 - Fish.
What is the precursor to vitamin D?
7-dehydrocholesterol.
What is vitamin D3 converted into?
25-hydroxyvitamin D3.
What role does the liver play in vitamin D synthesis?
It is responsible for the conversion of vitamin D3 into 25-hydroxyvitamin D3.
What role does the kidney play in vitamin D synthesis?
Which hormone stimulates the kidney to carry out this role?
- It is responsible for the conversion of 25-hydroxyvitamin D3 into 1,25-dihydroxyvitamin D3.
- PTH stimulates the kidney to do this.
Which form of vitamin D binds to the vitamin D receptor?
1,25-dihydroxyvitamin D3.
What is the function of vitamin D3?
To increase calcium and phosphate reabsorption at the intestine.
Which form of vitamin D is measured clinically?
25-hydroxyvitamin D3.
What is the normal adult reference range for vitamin D?
7.5 - 50 nmol/L.
List the names of each form of vitamin D by which they are commonly known.
- Vitamin D: Calciferol.
- 25-(OH) D: Calcidiol.
- 1,25-(OH)2 D: Calcitriol.
Which cells produce calcitonin?
Thyroid c-cells.
What stimulates calcitonin release?
Hypercalcaemia.
What is the action of calcitonin?
Inhibition of bone breakdown via direct effect on osteoclasts.
List 3 actions of parathyroid hormone on the kidney.
1 - Increases urinary phosphate.
2 - Decreases urinary calcium.
3 - Stimulates 1,25-hydroxyvitamin D3 production.
Where is fibroblast growth factor 23 produced?
Osteocytes and osteoblasts.
What stimulates fibroblast growth factor 23 production?
High serum phosphate.
List 2 functions of fibroblast growth factor 23.
1 - To increase renal excretion of phosphate.
2 - To suppress renal synthesis of vitamin D (further reducing serum phosphate).
Of which disease is high fibroblast growth factor 23 an indicator?
Renal disease.
List 3 non-collagenous proteins that are essential to bone function.
1 - Osteocalcin.
2 - Osteonectin.
3 - Osteopontin.
How is the extracellular matrix of bone calcified?
With the formation of hydroxyapatite crystals (containing both calcium and phosphate).
List 3 cell types within bone.
1 - Osteocytes.
2 - Osteoblasts.
3 - Osteoclasts.
What is the primary function of osteocytes?
Maintenance of the strength of the extracellular matrix of bone.
Describe the location and structure of osteocytes.
- They are embedded in the calcified bone matrix that they produce.
- They have a stellate shape with long processes that contact other osteocytes and osteoblasts.
From which cells do osteoblasts originate?
Mesenchymal stem cells.
List 2 functions of osteoblasts.
1 - To form the extracellular matrix of bone.
2 - To aid calcification.
What is the function of osteoclasts?
To reabsorb the contents of bone into the blood.
Where are osteoclasts found?
In contact with calcified bone surfaces (e.g. in lacunae / depressions).
How do osteoclasts reabsorb the contents of bone?
By producing acid (to resorb the minerals) and enzymes (to resorb the matrix).
Give an example of a histologically distinctive characteristic of osteoclasts.
They are multinucleated.
Via which proteins do osteoclasts attach to the bone surface?
Integrins.
Describe the process of bone remodelling.
1 - Reversal phase (creating an osteogenic environment at the apex of the lacuna).
2 - Osteoid formation (unmineralised bone).
3 - Mineralisation of the osteoid.
What is a metabolic bone disease?
Any of:
1 - Hyperparathyroidism.
2 - Rickets (osteomalacia).
3 - Renal osteodystrophy.
4 - Osteoporosis.
What might cause primary hyperparathyroidism?
A parathyroid tumour.
List 2 complications of hyperparathyroidism.
1 - Hypercalcaemia.
2 - Hypophosphataemia.
What might cause secondary hyperparathyroidism?
How?
- Renal disease.
- Renal disease would increase serum phosphate and decrease activation of vitamin D.
How can secondary hyperparathyroidism be treated?
- Phosphate binders.
- Vitamin D analogues.
What is tertiary hyperparathyroidism?
Where long-standing secondary hyperparathyroidism leads to irreversible parathyroid hyperplasia.
How can tertiary hyperparathyroidism be treated?
Surgery.
What is the difference between osteoporosis and osteomalacia?
- Osteomalacia is characterised by poor mineralisation of bone (leading to soft bone).
- Osteoporosis is characterised by low bone density, although the composition of bone is normal (leading to porous and brittle bone).
What is the difference between rickets and osteomalacia?
Osteomalacia in children is known as rickets.
Describe renal osteodystrophy.
- A renal disease resulting in osteomalacia (a cause of secondary hyperparathyroidism).
- Urinary phosphate decreases, urinary calcium increases and 1,25-(OH)2 D3 decreases.
List 3 causes of rickets / osteomalacia.
1 - Failure to absorb sufficient Ca2+ from the GI tract.
2 - Lack of sunlight.
3 - Lack of dietary Ca2+.
What is necessary to convert 7-dehydrocholesterol into vitamin D3?
UV light.
What causes the bowed legs seen with rickets?
A weak osteoid at the growth plates.
What causes the swollen joints seen with rickets?
Excess growth of growth plates as a compensatory mechanism for weak growth plates.
Where in a long bone are the growth plates found?
Between the epiphyses and metaphyses.
List 2 signs and symptoms of osteomalacia.
1 - Bone pain.
2 - Pseudofractures.
How is rickets / osteomalacia treated?
Vitamin D replacement.
What causes postmenopausal osteoporosis?
A decline in oestrogen, which increases the rate of bone remodelling and bone reabsorption (breakdown).
What are the long-term consequences of spinal osteoporosis?
A kyphotic spine.
What is the importance of phosphate?
It is necessary for bone calcification (as the hydroxyapatite crystals contain both calcium and phosphate).
