My Clinical Notes

• Derived from the pharangeal pouches which also give rise to the thymus
• Normally the four glands lie in close proximity to the upper and lower poles of each thyroid lobe but they may be found anywhere along the pathway of descent of the pharyngeal pouches
• 10% of people only have 2-3 glands
• Mostly composed of chief cells which produce PTH
• Activity of the gland is controlled by free calcium in the bloodstream. Low levels stimulate secretion of PTH
• The PTH receptor is a seven transmembrane G-protein coupled receptor

 

• Activity of PTH;
  • Stimulates osteoclasts thereby mobilising calcium from bone
  • Increases renal tubular calcium reabsorption
  • Increases convertion of vitamin D to its active dihydroxy form in the kidneys
  • Increases urinary phosphate excretion, thereby lowering serum phosphate levels
  • Augments gastrointestinal calcium absorption
  • The net result is an increase in serum calcium levels which inhibits further PTH secretion via a negative feedback loop

 

• Hypercalcaemia is one of the major changes induced by elevated levels of PTH
• It is a relatively common complication of malignancy
• The prognosis of patients with malignancy-associated hypercalcaemia is generally poor as it occurs in more advanced cancers
• The two major mechanisms by which it can occur when associated with malignancy is;
  • Osteolytic metastases
    • RANKL is secreted by tumour cells and by binding to the RANK receptor causes osteolysis by promoting the proliferation, differentiation, fusion and activation of osteoclasts
  • Release of PTH-related protein
    • The most frequent cause of hypercalcaemia in nonmetastatic solid tumours

This entry was posted on Sunday, December 30th, 2007 at 2:14 pm and is filed under Endocrine. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.