FHH is caused by a mutation in the extracellular calcium sensor protein which is found in the parathyroid glands. Normally, a rise in extracellular calcium activates the sensor protein and, via a phospholipase C mediated pathway, results in inhibition of parathyroid hormone secretion.
Mutations occur at the binding site for calcium on the extracellular domain of the sensor protein, resulting in decreased binding. Thus, patients with FHH have a raised "set point" for extracellular calcium homeostasis.
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