Marked hypomagnesemia also reduces PTH levels even in the presence of hypocalcemia; this phenomenon is referred to as “paradoxical blockade of PTH secretion.” With serum Mg below 1.2 mg/dL, there is a disinhibition of Gα-subunits, which mimics activation of the CaSR and reduction of PTH secretion (Quitterer et al., 2001; Vetter and Lohse, 2002). Here, PTH is linked to familial primary hypomagnesemia.