|Familial hypocalciuric hypercalcaemia (FHH)
- Parathyroid glands lie posterior to the thyroid
- They secrete PTH from chief cells
- Derived from 3 and 4 pharyngeal pouches
- Normal levels range from 10 to 65 pg/mL.
Stimulus to release of PTH
- G protein-coupled transmembrane receptor, calcium-sensing receptor (CaSR), on the surface of chief cells responds to low serum calcium and activates translation and secretion of PTH.
- A low Ionized calcium level increases PTH, Normal/High Ca stops PTH release
Effects of PTH
- Inhibits osteoblast activity and stimulates osteoclast activity
- Increased osteoclast activity releases Ca2+/PO4-3 from bones as PTH binds to cell receptors on osteoblasts stimulating the release of Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL). RANKL binds to its receptor on osteoclast precursors, stimulating them to fuse into mature osteoclasts thereby increasing calcium resorption from bone.
- Increased Ca uptake from kidney
- Decreased PO4 uptake from kidney
- Increased 1,25 Vitamin D
- Net effect is raised Calcium and lowers Phosphate