PTH and familial hypocalciuric hypercalcemia: Familial hypocalciuric hypercalcaemia (FHH) is an autosomal dominant disorder of extracellular calcium (Ca2+o) homeostasis characterized by lifelong mild-to-moderate elevations of serum calcium concentrations, mild hypermagnesaemia, normal or elevated circulating parathyroid hormone (PTH) concentrations and inappropriately low urinary calcium excretion [mean urinary calcium to creatinine clearance ratio (CCCR) <0.01] (1–4).