What are the actions of the parathyroid glands: decrease intestinal absorption of calcium, increase calcium reabsorption in the kidney, produce calcitonin, stimulate osteoblastic activity, or stimulate production of active Vitamin D (Vit D)?

Actions of the Parathyroid Glands

The parathyroid glands primarily increase calcium reabsorption in the kidney, stimulate the production of active vitamin D, and stimulate bone calcium release, but do NOT produce calcitonin or decrease intestinal calcium absorption.

Physiological Actions of Parathyroid Hormone (PTH)

Parathyroid hormone is the primary hormone secreted by the parathyroid glands and plays a crucial role in calcium homeostasis through three main mechanisms:

1. Increases calcium reabsorption in the kidney:

   • PTH binding to PTH1R in the distal and proximal renal tubules increases calcium absorption 1
   • This action helps maintain normal serum calcium levels by reducing urinary calcium excretion

2. Stimulates production of active vitamin D:

   • PTH stimulates the enzyme 1-α-hydroxylase (CYP27B1) in the kidneys 1
   • This enzyme hydroxylates 25-hydroxyvitamin D into 1,25-dihydroxyvitamin D (active form) 1
   • Active vitamin D then increases intestinal absorption of calcium 1, 2

3. Stimulates bone calcium release:

   • PTH binding to PTH1R in bones stimulates the release of calcium and phosphate from bones into circulation 1, 3
   • This increases serum calcium levels during hypocalcemic states

What Parathyroid Glands Do NOT Do

• Do NOT produce calcitonin:

  • Calcitonin is produced by parafollicular C cells in the thyroid gland, not the parathyroid glands 1
  • Calcitonin has opposing effects to PTH, decreasing serum calcium by inhibiting bone resorption

• Do NOT decrease intestinal absorption of calcium:

  • PTH indirectly increases (not decreases) intestinal calcium absorption by stimulating active vitamin D production 1, 4
  • Active vitamin D enhances calcium absorption in the intestines

Regulation of PTH Secretion

PTH secretion is regulated primarily by:

• Serum calcium levels: Hypocalcemia is sensed via calcium-sensing receptors (CaSR) on parathyroid glands, triggering PTH release 1
• Vitamin D levels: Low 1,25-dihydroxyvitamin D increases PTH secretion 1
• Phosphate levels: Hyperphosphatemia directly affects parathyroid gland function and increases PTH secretion 1

Clinical Significance

Understanding the actions of parathyroid glands is critical for managing disorders of calcium metabolism:

• In chronic kidney disease, decreased kidney function leads to phosphate retention, decreased active vitamin D production, and secondary hyperparathyroidism 1, 4
• Hypocalcemia management requires understanding the role of PTH in calcium regulation 5
• Hypoparathyroidism results in hypocalcemia due to insufficient PTH action on kidneys, bone, and vitamin D activation 6

Common Pitfalls in Understanding Parathyroid Function

• Confusing calcitonin (produced by thyroid C cells) with PTH (produced by parathyroid glands)
• Misunderstanding the indirect effect of PTH on intestinal calcium absorption (via vitamin D activation)
• Overlooking the multiple mechanisms by which PTH increases serum calcium (kidney, bone, and intestine via vitamin D)

The parathyroid glands serve as the body's calcium thermostat, maintaining tight regulation of serum calcium through these coordinated actions on multiple target organs.