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MEH > 11. Parathyoid Hormone > Flashcards

11. Parathyoid Hormone Flashcards

1
Q

Which important processes does calcium play a role in?

A
  • Neuromuscular excitability
  • Coagulation
  • Synaptic transmission
  • Exocytosis
  • Intracellular second messenger
  • Regulation of gene transcription
  • Bone formation
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2
Q

What important roles does phosphate play?

A

Part of ATP - cellular energy metabolism

Activation and de-activation of enzymes

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3
Q

Is phosphate concentration regulated tightly?

A

No, levels fluctuate during the day, particularly after meals.

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4
Q

Where is the bodies largest store of calcium, how is it stored?

A

Skeleton bones - as hydroxyapatite crystals (contain phosphate and calcium)

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5
Q

Which 3 hormones are involved in regulating calcium and phosphate levels?

A
  1. Parathyroid hormone (PTH)
  2. Calcitriol
  3. Calcitonin (less important)
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6
Q

Which 3 organ systems do these hormones act on?

A

Bone
Kidneys
GI tract

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7
Q

What 3 forms does calcium exist in within the plasma?

A
  1. Free ionised
  2. Protein- bound (albumin)
  3. Complexed with organic ions (citrate, oxalate)
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8
Q

What is the total concentration of all 3 calcium forms in the plasma?

A

2.2-2.7 mmol/L

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9
Q

Which form is the most important in terms of regulating PTH secretion?

A

Free ionised form - physiologically active

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10
Q

What symptoms are associated with hypocalcaemia?

A

Hyper-excitability of the NS - parasethesia of mouth and fingers, tetany, paralysis, convulsions

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11
Q

What symptoms are associated with hypercalcaemia?

A

Groans - constipation
Stones - kidney stones
Moans - depression, tiredness and dehydration
Bones - bone and muscle aches

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12
Q

What is calcitriol?

A

Active form of vitamin D

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13
Q

How do the effects of PTH and calitrol compare?

A

Both raise serum calcium, but via different mechanisms and over different time scales.
PTH = short term
Calcitriol = longer term

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14
Q

What are the 2 forms of Vitamin D?

A 
D2 = ergocalciferol
D3 = cholecalciferol
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15
Q

What reactions must inert Vitamin D2/D3 undergo in order to become activated?

A

2 hydroxylation reactions in the liver and kidney

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16
Q

How can vitamin D obtained within the body?

A

D3 Synthesis in skin when sun exposure and dairy

D2 - yeast, fungi, margarines

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17
Q

How can tumour influence calcium metabolism?

A

Some tumours produce parathyroid hormone related peptide (PTHrP) which is a PTH analogue and causes hypercalcaemia.

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18
Q

Which cancers commonly produce PTHrP?

A

Breast, prostate and occasionally myeloma.

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19
Q

How do the effects of PTHrP differ to PTH itself?

A

Does not increase renal C-1 hydroxylase activity so does not increase calcitriol concentration like PTH.

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20
Q

How does PTHrP cause hypercalcaemia?

A

Shares actions of PTH leading to increased calcium release from bone, reduced renal excretion and reduce phosphate reabsorption.

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21
Q

What are the actions of PTH?

A

Stimulates osteoclasts and bone resorption, increased calcium release.
Stimulates renal calcium reabsorption
Stimulates excretion of phosphate
Stimulates C-1 hydroxylase activity which activates VitD to calcitriol.

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22
Q

What is the effect of calcitonin?

A

Counteract PTH - decrease calcium levels

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23
Q

Where is calcitonin released from?

A

Thyroid gland - parafollicular C cells

24
Q

Where are the parathyroid glands, how many do most people have?

A

Posterior thyroid

Usually 4

25
Q

What are the 2 cell typespresent in the parathyroid gland?

A

Chief cells - secrete PTH

Oxyphil cells

26
Q

What type of hormone is PTH?

A

polypeptide hormone, water-soluble, no binding protein.

27
Q

What stimulus up-regulates PTH synthesis?

A

low serum Ca2+

28
Q

What is the T1/2 of PTH?

A

4 minutes - short term

29
Q

How do serum calcium levels alter PTH synthesis?

A

When plasma calcium is high, Ca2+ binds to GPCR alpha q coupled receptor.
IP3 second messenger reduces PTH synthesis inhibits PTH secretion.

30
Q

Why is it important that phosphate excretion is increased when calcium is released?

A

Precent formation of hydroxyapatite crystals within the body

31
Q

What does calcitriol do?

A

Increased calcium uptake in the GI tract.

Increases kidney reabsorption of calcium

32
Q

How does the half life of Vitamin D3 and calcitriol differ to PTH?

A

Longer half life, hence longer term regulation of calcium levels.

33
Q

What protein is vitamin D3 bound to in the plasma?

A

Transcalciferin

34
Q

What role does calcium play in the clotting cascade?

A

Factor IV

35
Q

What is EDTA used for?

A

Calcium chelator used to prevent blood samples clotting

36
Q

Which patients are likely to need IV calcium?

A

Patients given >5 units of blood

37
Q

What are the most common causes of hypercalcaemia in the hospital setting?

A
  • Malignant osteolytic bone metastases

- Multiple myeloma

38
Q

Which common cancers metastasise to bone causing LYTIC lesions and hypercalcaemia?

A

Breast
Kidney
Thyroid
Lung

39
Q

Prostate cancer metastasises to bone but doesn’t cause hypercalcaemia, why?

A

Osteoblastic effect rather than lytic.

40
Q

What are the most common sites for bone metastases?

A 
Vertebrae
Pelvis
Ribs
Skull
Proximal humerus and femur
41
Q

What causes primary hyperparathyroidism?

A

PTH secreting- adenoma of one of the parathyroid gland

42
Q

What would you expect serum calcium and phosphate levels to be in a patient with primary hyperthyroidism?

A

Raised calcium

Low phophate

43
Q

What is secondary hyperparathyroidism usually due to?

A

Vitamin D deficiency

44
Q

What can cause vitamin D deficiency?

A
  • Dietary deficiency

- Chronic renal failure - cannot hydroxylate and activate

45
Q

What would you expect serum calcium and PTH levels to be in patients with secondary hyperparathyroidism?

A 
Raised PTH (negative feedback)
Low calcium
46
Q

What condition affecting the bone is often the presenting cause of Vitamin D deficiency?

A

Osteomalacia - patients experience bone pain due to renal osteodystrophy

47
Q

How does calcium affect neuronal activity?

A

Raises the threshold for membrane depolarisation

48
Q

What effect does hypercalcaemia have on neuronal activity?

A

Suppresses neuronal activity - lethargy, confusion, coma

49
Q

What effect does hypocalcaemia have on neuronal activity?

A

tingling, tetany, convulsions.

Can kill due to laryngeal muscle tetany.

50
Q

What is the difference between osteoporosis and osteomalacia?

A

Osteoporosis - decreased bone density but normal ratio of mineral to matrix. (normal bone, just less of it)
Osteomalacia - decreased ratio of mineral to matrix

51
Q

What condition is a result of osteomalacia in children?

A

Rickets

52
Q

What are risk factors for osteoporosis?

A 
Post-menopausal women
low BMI
Long term steroid used
Prolonged inactivity
Heavy drinking
53
Q

What is high alkaline phosphatase an indicator of?

A

Enzyme on osteoblasts, marker of bone turnover. Increased in hyperparathyroidism.

54
Q

Which 2 ways can malignancy lead to hypercalcaemia?

A
  1. Bone metastases produce
    local factors that act in a paracrine manner to activate osteoclasts (osteolytic)
  2. Produce PTHrP that acts on PTH receptors
55
Q

Parathyroid hormone has direct actions on the gastrointestinal tract to increase the rate of clacium and phosphate absorption from food. True or False?

A

False - indirect through promoting formation of calcitriol

56
Q

What type of hormone is calcitonin?

A

Peptide

57
Q

7-dehydrocholesterol is converted to vitamin D3 by?

A

Sunlight

MEH (19 decks)

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