What is the effect of Parathyroid Hormone (PTH) on calcium metabolism except?

Effects of Parathyroid Hormone (PTH) on Calcium Metabolism

The correct answer is D. Decrease Ca resorption from bone.

PTH increases calcium resorption from bone rather than decreasing it, making option D incorrect. PTH has three major actions on calcium metabolism: increasing calcium resorption from bone, increasing calcium reabsorption in the kidney, and enhancing calcium absorption from the intestine (indirectly through vitamin D activation) 1, 2.

Physiological Actions of PTH on Calcium Metabolism

PTH is the primary hormone responsible for maintaining calcium homeostasis through several mechanisms:

1. Bone Effects:

   • PTH stimulates osteoclastic bone resorption, liberating calcium and phosphate from the skeleton 2
   • This increases serum calcium levels by mobilizing calcium from bone storage
   • This effect occurs over hours to days 2

2. Kidney Effects:

   • PTH rapidly increases renal tubular calcium reabsorption 2
   • It decreases phosphate reabsorption in the proximal tubule (phosphaturic effect)
   • PTH stimulates 1-alpha-hydroxylase in the kidney to convert 25(OH)D to active 1,25(OH)2D (calcitriol) 1

3. Intestinal Effects (indirect):

   • PTH indirectly enhances intestinal calcium absorption by increasing 1,25(OH)2D production 2
   • The increased 1,25(OH)2D then augments intestinal calcium absorption 3

Regulation of PTH Secretion

PTH secretion is primarily regulated by serum ionized calcium levels through the calcium-sensing receptor (CaSR) on parathyroid chief cells:

• Hypocalcemia → decreased CaSR activation → increased PTH secretion
• Hypercalcemia → increased CaSR activation → decreased PTH secretion 2

Clinical Implications

Understanding PTH's effects on calcium metabolism is crucial for managing disorders such as:

1. Secondary Hyperparathyroidism in CKD:

   • Requires monitoring of PTH levels to detect skeletal and mineral disorders 1
   • Treatment includes dietary phosphate restriction, phosphate binders, vitamin D supplementation, and sometimes calcimimetics like cinacalcet 1

2. Post-Bariatric Surgery:

   • Decreased calcium and vitamin D absorption can lead to secondary hyperparathyroidism 1
   • Regular PTH monitoring is recommended to detect calcium and vitamin D deficiency 1

3. Post-Thyroidectomy:

   • PTH measurement helps predict postoperative hypoparathyroidism and hypocalcemia 1

Common Pitfalls in Understanding PTH Physiology

1. Confusing PTH with PTHrP: While PTH is produced by the parathyroid glands, PTH-related protein (PTHrP) is produced by certain cancers and can cause humoral hypercalcemia of malignancy 4

2. Overlooking the dual effect on bone: While the net effect of PTH is to increase calcium resorption from bone, intermittent PTH administration can actually have anabolic effects on bone, which is the basis for its use in osteoporosis treatment 3

3. Assuming PTH is the only regulator of calcium: The calcium-sensing receptor also directly regulates renal calcium handling independent of PTH 5

In summary, PTH increases (not decreases) calcium resorption from bone, increases renal calcium reabsorption, and enhances intestinal calcium absorption through vitamin D activation.