PTH and hypoparathyroidism: PTH regulates 1-a-hydroxylase in the kidney, which is the enzyme that converts 25-hydroxy-vitamin D to its biologically active form, 1,25-dihydroxy-vitamin D. Thus, an explanation for the lack of association between vitamin D deficiency and post-thyroidectomy hypocalcemia is that hypoparathyroidism results in decreased conversion of 25-hydroxy-vitamin D to 1,25-dihydroxy-vitamin D, regardless of the amount of 25-hydroxy-vitamin D available.