PTH and Hypocalcemia: PTH is a major regulator of renal 1α-hydroxylase (CYP27B1) activity, thereby increasing the level of active metabolites of vitamin D (1,25-dihydroxyvitamin D; calcitriol) in the circulation; its absence, as in hypoparathyroidism, results in low serum calcium and ultimately manifests itself as symptomatic hypocalcemia [10].