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Endocrinology > Bone Metabolism > Flashcards

Bone Metabolism Flashcards

1
Q

How are calcium stored in bones?

A

hydroxyapatite crystals (99%…1% as free)

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2
Q

What are the acute and chronic responses to hypocalcemia?

A
  • acute – release from rapid exchange pool (minutes)
    - regulated by osteocyte (osteocytic osteolysis)
  • chronic – resorption of bone (requires days)
    - regulated by osteoclast
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3
Q

What are the acute and chronic effects of PTH?

A

Bone – acute release rapid pool Ca++,
then chronic resorption
Kidney – increased Ca++ resorption
- decreased PO4 (phosphate) resorption (increased ionized Ca2+)
- increased 1,α hydroxylase (required for synthesis of active form of VitD3 [1,25-dihydroxycholecalciferol]
Gut –> effect only via VitD3 facilitates absorption of Ca2+ and PO4-

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4
Q

Where is calcitonin made and what is its releveance in humans?

A

Made in C-cells [parafollicular cells] in thyroid and is stimulated by hypercalemia –> reduces Ca/PO4 resorption in bone and kidneys

Human relevance:
Useful as marker for medullary thyroid cancer
Excess can cause diarrhea eg medullary cancer

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5
Q

Where is calcium sensing receptors located?

A

parathyroid

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6
Q

What the effect of 1, 25 - VitD3 on the gut?

A

increased Ca and PO4 absorption (for phosphate it increases from 65% absorption to 85/90%)

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7
Q

What does high calcium do to PTH release?

A

turns off PTH release

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8
Q

What type of phosphates exist in the body?

A

85% as hydroxyapatite crystals; 12% bound to protein;

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9
Q

What two processes promote VitD3 production and 1-alpha hydroxylase

A

hypocalcemia, and hypophosphatemia

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10
Q

From where is FGF-23 released and in response to what?

A

released by osteocytes and osteoblasts in response to high phosphates and high VitD3

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11
Q

What does FGF-23 do?

A
  • suppress PTH release
  • degrade renal NaPT2 cotransporter (reduces phosphate reabsorption)
  • inhibit alpha-hydroxylase
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12
Q

What is Hypophosphatemic Diseases?

A
  • rare disease where FGF-23 is not broken down (as a result to mutation to PHEX) or increased FGF-23 due tumor
  • leads to fractures
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13
Q

What are the two fracture risk calculators?

A

CAROC or FRAX

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14
Q

What is bone remodelling?

A

formation and resorption

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15
Q

Which group of bone cells help in formation, and resorption respectively?

A

osteoblast - formation - 3 months

osteoclasts - resorption - 3 weeks

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16
Q

What is the physiological trigger for bone formation?

A

microfracture

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17
Q

What are osteocytes and their function?

A

Osteoblasts that stay in the bone and become osteocytes:

  • functions to sense microfractures
  • inhibits formation of osteoblasts
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18
Q

Which cells express PTH and VitD receptors, and alkaline phosphatases?

A

osteoblasts (ie. release FGF-23 if VitD3 or phosphate high)

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19
Q

Whats the role of osteoblasts?

A

synthesize and secrete organic matrix

- also regulates osteoclast formation and activation (via RANKL and osteoprotregin)

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20
Q

What cells are multinucleated and aid in Ca resorption?

A

osteoclast (from hematopoietic cells)

  • secretes enzymes that dissolve bone (cathepsin K)
  • RANKL from osteoblasts stimulte their development
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21
Q

What are some anti-resorptives?

A

Those that stimulate osteoprotegrin (inhibits osteoclasts):

- Estrogen, Testosterone, calcitonin, bisphosphonates, IFN gamma

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22
Q

What are pro-resorptives

A

Those that potentiates RANKL (stimulates osteoclasts):

-PTH, 1, 25 VitD3, corticosteroids, IL-6

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23
Q

What are some signs and symptoms of hypercalcemia?

A

CNS effects – confusion, lethargy
- hypotonia, hyporeflexia

Gut effects – constipation, nausea, vomiting, anorexia, abdominal pain

Renal effects – polyuria (osmotic diuresis)
- chronic – renal stones, nephrocalcinosis

Heart – arrhythmias, asystole
(short Q-T time on EKG)

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24
Q

What are the 3 most common causes of hypercalcemia?

A

1) Primary hyperparathyroidism
- Excess PTH from autonomous benign adenomas (80% only one gland, 20% all four [familial])

2) Malignant disease
- tumors make PTH-related protein (PTH-RP) and other osteoclast-activating substances (RANKL)
- PTH-RP binds to PTH receptors and mimics PTH action exactly

3) Granulomas (sarcoidosis, TB)
- have 1-alpha hydroxylase and convert 25 (OH)-vit. D3 to 1,25 (OH)2 D3

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25
Q

What are some rare causes of hypercalcemia?

A

1) Vitamin D overdose – increased gut absorption
2) Thyrotoxicosis – increased bone turnover
3) Thiazide diuretics – increased calcium reabsorption
4) Familial hypocalciuric hypercalcemia (FHH) [RARE]
- inherited deficiency of calcium level sensor protein, resulting in hypercalcemia (new set point)
- receptor mutations cause decreased sensitivity to calcium levels
5) Milk alkali syndrome
Diuresis and water and Na+ loss, which leads to increased bicarbonate reabsorption and alkalosis.
Alkalosis enhances calcium reabsorption

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26
Q

How to calculate corrected serum?

A

For every 10 g albumin decrease below 40 g/L, add 0.2 mmol/L serum calcium
eg albumin = 30 g/L, serum calcium = 2.0 mmol/L
Thus corrected calcium = 2.2 mmol/L

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27
Q

What do a low serum PTH, and high serum PTH usually mean in the context of hypercalcemia?

A
  • low serum PTH malignancy

- high serum PTH hyperparathyroidism

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28
Q

What is usually the treatment for primary hyperparathyroidism?

A
  • single parathyroid adenoma removed
  • near total thyroidectomy because of nodules
  • bisphosphonates (only protects bones but not calcium levels)
  • calcimimetics (binds to Ca sensing receptors leading to decreased PTH release)
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29
Q

True or False: cardiac arrythmia may cause death in primary hyperparathyroid patients

A

True

30
Q

What are some lab test findings with primary hyperparathyroidism?

A
  • high PTH, high calcium

- hypercalcemia, hypophophatemia, hyper- calciuria/phophaturia

31
Q

What are some common causes of hypocalcemia?

A

1) Inadequate PTH
- hypoparathyroidism (post-surgical [usually transient] or idiopathic)
- low magnesium (needed for PTH release) [due to malnutrition/alcoholism]

2) Inadequate VitD3
- gut malabsorption, poor intake, chronic renal failure, high phosphate (via FGF-23)

32
Q

What is the temporary and long-term treatment (for non-transient) surgical hypoparathyroidism?

A
  • transient: IV Ca

- permanent: Ca + VitD3

33
Q

What are some hallmarks for secondary hyperparathyroidism?

A
  • high PTH, high Ca, low VitD3

- asymptomatic

34
Q

What are some symptoms of hypocalcemia?

A
  • numbness, hand cramps, tingling of mounth and fingers, muscle spasms, tetany
  • Chvostek’s and Trousseau’s (for moderate to severe hypocalcemia)
35
Q

What are some common causes of hypomagnesemia and what is treatment?

A
  • decreased absorption – poor diet intake eg alcoholism
    • gut malabsorption
    • increased gut losses – diarrhea, vomiting, nasogastric suction
  • treatment: IV magnesium
36
Q

What are some hallmarks of secondary hypoparathyroisism?

A
  • low PTH, low/normal calcium, low VitD3, low magnesium

- VitD3 is low because low PTH results lower gut absorption of it (and additional malabsorption)

37
Q

What would be the emergency therapy of hypercalcemia?

A
  • Hydrate with saline – patient is dehydrated from osmotic diuresis induced by hypercaluria; Na+ in saline exchanges with Ca++ in kidney to promote calciuria
  • Furosemide – causes Ca++ excretion in kidney tubules
  • Bisphosphonates I.V. – pamidronate – inhibits bone resorption caused by PTH-RP; current most potent drug is zoledronate
  • Calcitonin I.V. – faster action but smaller effect on bone resorption than bisphosphonates
  • Hemodialysis – fastest therapy; should be used in critical emergencies
38
Q

What is osteomalacia?

A

increase in proportion of unmineralized bone (osteoid)

39
Q

What is osteoporosis?

A

decrease in total bone [mineralized and unmineralized] (osteoid- normal proportion)

40
Q

What is osteomalacia caused by?

A

Inadequate substrate (calcium, phosphate) and/or vitamin D (to stimulate calcium/phosphate absorption) to allow normal mineralization of osteoid

41
Q

True or False: osteoporosis is multifactorial?

A

True:

  • low peak bone mass – mostly genetically determined
  • decreased gut calcium absorption with aging
  • decreased 1,25 (OH)2 D3 formation
  • decreased renal calcium reabsorption with aging
  • hormonal – decreased E2, T, IGF-1 (these are anti-resorptives)
42
Q

Provide reasons why females are at a higher risk of fracture:

A

Lower peak bone mass
Loss of sex steroids (E2); men gradually have T decline
Women lose bone by loss of connecting arms in trabecular bone, men have thinning of arms
- structurally weaker
Women have higher endosteal resorption without much periosteal deposition of bone
- men have more periosteal deposition- T effect

43
Q

How would assess risk in a individual suspected of osteopenia?

A

Targeted history and physical
- clinical risk factors
- measurement of height (loss implies vertebral fractures)
- get up and go test (mobility and balance)
BMD – calculate risk category using CAROC or FRAX
Vertebral spine xrays

44
Q

What is the bone mineral density (BMD) values for osteopenia?

A

BMD between -1 and -2.5 SD below young adult mean

45
Q

What is the bone mineral density (BMD) values for osteoporosis?

A

BMD < -2.5 SD below young adult mean

46
Q

What is severe osteoporosis?

A

BMD < -2.5 SD below young adult mean

additional “fragility” fractures

47
Q

What tool is used to asses bone mineral density?

A

Dual x-ray absorptiometry (DEXA)

48
Q

What are the clinical risk factors for fractures?

A

Fragility fracture after the age of 40
Parental history of hip fracture
Premature menopause
Glucocorticoid use (> 7.5 mg/d) 3 months or more in the prior year (resorptive)
Lifestyle factors: smoking, excessive alcohol (> 3 units/d), and physical inactivity
Weight loss since age 25 >10%
Poor nutrition, calcium intake, vitamin D status
Recurrent falls, gait and balance
Low bone density

49
Q

What is a fragility fracture?

A

A fracture occurring spontaneously or following minor trauma such as a fall from standing height or less.
***Excluding craniofacial, hand, ankle and foot fractures

50
Q

Who should go for BMD testing?

A

All women and men age > 65

Postmenopausal women, and men aged 50 – 64 with clinical risk factors for fracture.

51
Q

What disorders can contribute to increased risk of fracture?

A

Primary hyperparathyroidism
Type I diabetes
Osteogenesis imperfecta
Untreated long-standing hyperthyroidism, hypogonadism, orpremature menopause (< 45 years)
Cushing’s disease
Chronic malnutrition or malabsorption
Chronic liver disease
Chronic obstructive pulmonary disease (COPD)
Chronic inflammatory conditions (e.g., rheumatoid arthritis [RA], inflammatory bowel disease)

52
Q

True or False: FRAX and CAROC take into account pelvic fractures and undiagnosed vertebral fractures.

A

False and so underestimates the total osteoporotic burden

53
Q

What factors increase CAROC basal risk by one category (ie low to moderate)?

A

Fragility fracture after age 40

Recent prolonged systemic glucocorticoid use

54
Q

What are some risk factors taken into account for FRAX?

A 
Parental hip fracture
Prior fracture
Glucocorticoid use
Current smoking
High alcohol intake
Rheumatoid arthritis
55
Q

What are the treatment option for low, moderate, and high risk groups for fracture?

A 
Low and moderate risk:  
        -   lifestyle modification
        -   take adequate calcium and vitamin D
High risk:  
        - pharmacotherapy
56
Q

What biochemical lab tests should be sought for an individual suspected of osteoporosis?

A 
Calcium, corrected for albumin 
Complete blood count
Creatinine
Alkaline phosphatase
Thyroid stimulating hormone (TSH)
Serum protein electrophoresis for patients with vertebral fractures
25-hydroxy vitamin D (25-OH-D)
57
Q

What are Looser’s Lines?

A

Pseudofractures seen in osteomalacia as seen in plain x-rays

58
Q

What instrument would give a definite diagmosis for osteomalacia?

A

bone biopsy

59
Q

What are the symptoms and signs for osteomalacia?

A

– adult – diffuse aching pain – rare
- child – failure to grow, short stature,
bony deformities
- signs – hypotonia, decreased reflexes, muscle weakness (similar to hypercalcemia)

60
Q

List the causes of osteomalacia:

A

A) Lack of vitamin D (rickets)

 - inadequate diet – malnourished, elderly
 - inadequate sunlight
 - *gut malabsorption eg small bowel disease – Crohn’s, celiac 								disease
 - lack of renal 1,25 (OH)2D3 synthesis
              - renal failure
              - anticonvulsants

B) Phosphate deficiency states
- dietary (rare); hereditary states (rare)

61
Q

What are some therapies for osteoporosis?

A

Nutritional – increase vitamin D to 800-1000 units / day
Pharmacotherapy choices:
- anabolic (growth stimulating): teriparatide (PTH)
- antiresorptive: bisphosphonates, calcitonin,
estrogen, raloxifene, denosumab

62
Q

True or False: Raloxifene can be used for non-vetebral fractures

A

False: only for vertebral

63
Q

What drugs can be used for vertebral, non-vertebral, and hip fractures?

A

Bisphosphonates, denosumab, estrogen (for menopausal)

64
Q

Which type of fractures can be treated with Teriparatide?

A

verterbral, and non-vertebral

65
Q

What are the mechanism of action of raloxifene?

A

– estrogen receptor modulators

  • estrogenic actions: on bone, blood clotting factors
  • antiestrogenic actions – on breast and uterus
66
Q

What is the mechanism of action for bisphosphonate?

A
  • inhibit resorption
  • promote osteoclast apoptosis - stored in bone (may cause atypical femoral fractures, jaw osteonecrosis) –> inhibits bone formation by slowing down bone turnover
67
Q

What is the mode of action for Denosumab?

A

Denosumab – monoclonal antibody to RANKL

                 - inhibit osteoclasts from forming
                 - may have same long-term use side effects as bisphosphonates
68
Q

True or False: Cathepsin K inhibitors withdrawn from market?

A

True: increased stroke

69
Q

What is the therapy for Paget’s Disease?

A
  • Usually none required (treatment)
  • Bisphosphonates
    - 50-96% remission of disease (alk. Phosphatase decrease) after 2-6 mo therapy; 50% pain, functional improvement
    - *zoledronate (most effective), alendronate, risedronate
70
Q

What is renal osteodystrophy?

A

decreased renal function and renal mass

71
Q

What diseases can result from osteodystrophy?

A

(1) osteitis fibrosa: secondary hyperparathyroidism and marrow fibrosis (60%)
(13% mixed osteitis + osteomalacia)
(2) osteomalacia: - inadequate calcitriol
(3) adynamic bone (40%) – no formation: static

Endocrinology (16 decks)

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