PATHOLOGY

Detailed explanation-1: -Starting at the kidneys, PTH stimulates the production of 1alpha-hydroxylase in the proximal convoluted tubule. This enzyme, 1alpha-hydroxylase, is required to catalyze the synthesis of active vitamin D-1, 25-dihydroxycholecalciferol from the inactive form 25-hydroxycholecalciferol.

Detailed explanation-2: -Vitamin D is converted enzymatically in the liver to 25-hydroxyvitamin D (25[OH]D), the major circulating form of vitamin D, and then in the kidney to 1, 25-dihydroxyvitamin D, the active form of vitamin D.

Detailed explanation-3: -However, 25-hydroxyvitamin D is thought to be largely inactive because it requires conversion to the active hormone 1, 25-dihydroxyvitamin D, also known as calcitriol. Calcitriol is produced by the kidney and extrarenal tissues that express 1-hydroxylase.