Is it correct that when Parathyroid Hormone (PTH) is released into the circulation, it causes the kidneys to absorb calcium and excrete phosphorus, stimulates Vitamin D to retain calcium and phosphorus in bones, and increases calcium and phosphorus absorption in the intestines?

PTH Physiology: Correcting Key Misconceptions

Your understanding contains both correct and incorrect elements—most critically, PTH does NOT stimulate vitamin D to "retain" calcium and phosphorus in bones; rather, PTH stimulates vitamin D production to enhance intestinal absorption of these minerals, and PTH itself mobilizes calcium FROM bone, not into it. 1, 2

Accurate PTH Actions on the Kidneys

Correct aspects:

• PTH causes the kidneys to reabsorb calcium (reducing urinary calcium loss) 1, 2
• PTH causes the kidneys to excrete phosphorus in the urine (phosphaturic effect) 3, 1

Mechanism:

• PTH binds to PTH1R receptors in the proximal tubule, decreasing phosphate reabsorption and leading to phosphaturia 3
• Simultaneously, PTH increases calcium reabsorption in both proximal and distal tubules 3, 2

PTH and Vitamin D Relationship: A Critical Correction

Your statement is incorrect. PTH does not stimulate vitamin D to "retain" minerals in bone. Here's what actually happens:

• PTH stimulates the conversion of 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D (active vitamin D) in the kidney 3, 4
• This active vitamin D then increases intestinal absorption of both calcium and phosphate from the gut 3, 1
• PTH and vitamin D form a tightly controlled feedback cycle, with vitamin D exerting negative feedback on PTH secretion 4

PTH Effects on Bone: The Opposite of What You Stated

Major misconception to correct:

• PTH mobilizes calcium FROM bone, not into it 1, 2
• PTH increases bone resorption and turnover to release calcium and phosphate into circulation 1, 2
• PTH acts via a Na-Ca exchange mechanism on bone cells to stimulate calcium release 5
• The primary effect is increased bone resorption, particularly in cortical bone, to maintain serum calcium 2

PTH Effects on Intestinal Absorption: Indirect Mechanism

Partially correct, but mechanism is wrong:

• PTH does NOT directly act on the intestines 1
• Instead, PTH indirectly increases intestinal calcium and phosphate absorption by stimulating active vitamin D production 3, 1
• The active vitamin D (1,25-dihydroxyvitamin D) is what directly increases intestinal calcium absorption by binding to vitamin D receptors 3

Summary of Correct PTH Physiology

The accurate sequence:

1. Kidney effects: Increases calcium reabsorption, decreases phosphate reabsorption (causing phosphate excretion) 3, 1
2. Vitamin D activation: Stimulates conversion to active 1,25-dihydroxyvitamin D 3, 4
3. Intestinal effects (indirect): Active vitamin D increases calcium and phosphate absorption from gut 3, 1
4. Bone effects: Mobilizes calcium and phosphate FROM bone through increased resorption 1, 2

Net result: All three mechanisms work together to increase serum calcium levels, which is PTH's primary physiologic goal 2, 4