Familial primary hypomagnesemia with normocalciuria and normocalcemia (FPHNN) is a form of familial primary hypomagnesemia (FPH), characterized by low serum magnesium (Mg) values but inappropriate normal urinary Mg values (i.e. renal hypomagnesemia). The typical symptoms are weakness of the limbs, vertigo, headaches, seizures, brisk tendon reflexes and mild to moderate psychomotor delay.