Although this presentation will focus on vitamin D, it is critical to understand the control of parathyroid hormone (PTH) as well as its vital role in controlling the vitamin D synthetic pathway. Parathyroid hormone can be considered the conductor of the calcium and phosphate balance symphony, with vitamin D and its metabolites one of the sections of the symphony.
1. A decrease in extracellular (plasma or serum) calcium stimulates the release of parathyroid hormone.
a. PTH stimulates bone resorption thus helping to restore plasma calcium.
b. PTH also affects the kidney.
i. Increases 1,25(OH)2Vitamin D production (see next page).
ii. Directly increases calcium reabsorption.
iii. Decreases phosphate reabsorption (it is phosphaturic).
2. The primary negative feedback effect on the parathyroid gland is the restoration of plasma calcium to normal. However, 1,25(OH)2Vitamin D does have a small inhibitory effect on the parathyroid gland.