25. Calcium & Phosphate Metabolism Flashcards
Bone turnover serves as homeostasis of serum calcium and phosphate, in conjunctions with what other controls?
- Parathyroid hormone (PTH)
- Vitamin D (1,25-dihydroxy D3)
- Calcitonin
- FGF-23
Briefly, describe calcium homeostasis
- 99% of body calcium is in bone
- Remaining 1% is mainly intracellular
- Hormonal control of the tiny (<0.1%) extracellular fraction is what maintains Ca balance
- Extracellular: plasma Ca 2.2-2.6 mmol L-1
- About half is free [Ca2+] (physiologically active), half protein bound (mainly albumin)
Describe phosphate homeostasis
- 85% of body phosphorus is in bone
- Remainder is mainly intracellular
- Extracellular: H2PO4-, HPO42-, 2.5-4.5 mg dL-1 (0.75-1.45 mmol L-1)
- May fluctuate more than Ca
List some clinical features of hypercalcaemia
- Depression, fatigue, anorexia, nausea, vomiting,
- Abdominal pain, constipation
- Renal calcification (kidney stones)
- Bone pain ~ “painful bones, renal stones, abdominal groans, and psychic moans,”
Severe: cardiac arrhythmias, cardiac arrest
List some causes of hypercalcaemia.
COMMON:
- In ambulatory patients: primary hyperparathyroidism
- In hospitalized patients: malignancy
LESS COMMON:
- Hyperthyroidism
- Excessive intake of vitamin D
Describe the serum biochemistry of hyperparathyroidism.
- Serum calcium - modest to marked increase
- Serum phosphate - low or low normal
- Serum alkaline phosphatase raised in ~ 20% of cases
- Serum creatinine may be elevated in longstanding disease (kidney damage)
- Serum PTH concentration should be interpreted in relation to calcium
Describe hypercalaemia of malignancy.
Most common cause of hypercalcaemia in hospitalized patients
- Humoral, e.g., lung carcinoma secreting PTHrP
- Metastatic
Haematological
- myeloma
List some causes of hypocalcaemia
COMMON CAUSE:
- Vitamin D deficiency
- Renal failure
LESS COMMON CAUSES:
- Hypoparathyroidism
What is the difference between rickets and osteomalacia?
- Bone disease associated with vitamin D deficiency
- Rickets - in children, failure of bone mineralisation and disordered cartilage formation
- Osteomalacia - in adults, impaired bone mineralisation
What are some features of osteomalacia?
- Diffuse bone pain
- Waddling gait, muscle weakness
- On X-ray, stress fractures
Serum biochemistry:
- Low/normal calcium
- Hypophosphataemia
- Raised alkaline phosphatase
- Secondary hyperparathyroidism
What is the difference between osteoporosis and osteomalacia?
Osteoporosis: loss of bone mass:
- Endocrine
- Malignancy
- Drug-induced
- Renal disease
- Nutritional
Osteomalacia: loss of bone mineralization
How would you diagnose osteoporosis?
Measurement of bone mineral density (BMD). Dual-energy X-ray absorptiometry (DEXA or DXA scan)
T score
- Number of SDs below average for young adult at peak bone density
Z score
- Matched to age and/or group
List some endocrine causes of osteoporosis
- Hypogonadism – notably any cause of oestrogen deficiency
- Excess glucocorticoids – endogenous or exogenous
- Hyperparathyroidism
- Hyperthyroidism
What are some treatments for osteoporosis?
- Postmenopausal: HRT – effects well established but safety of long term treatment has been questioned
- Bisphosphonates – inhibit function of osteoclasts: risedronate, alendronate
- PTH analogues
- Denosumab – antibody against RANK ligand
- Ensure adequate calcium and vit D intake, appropriate exercise
After long research and study, what are the modified HRT guidelines?
- Short-term therapy (3-5 years) for treating vasomotor symptoms
- Lowest effective dose to be used
- Long term use not recommended