Calcium and Phosphate Homeostasis

Calcium and phosphate homeostasis is essential for bone integrity, neuromuscular function, and cellular signaling. The primary regulators are parathyroid hormone, vitamin D, and calcitonin.

Parathyroid Hormone (PTH)

PTH is released by the parathyroid glands in response to low serum calcium. It raises calcium levels by promoting bone resorption, increasing renal calcium reabsorption, and stimulating the activation of vitamin D.

Vitamin D

Vitamin D, activated via PTH, enhances the intestinal absorption of both calcium and phosphate. This action supports elevated blood calcium and phosphate for bone mineralization.

Calcitonin

Calcitonin is secreted by thyroid C cells during hypercalcemia. It lowers blood calcium by inhibiting osteoclast-mediated bone resorption.

Hypocalcemia and Hypercalcemia

Hypocalcemia results in neuromuscular excitability, tetany, and seizures. Hypercalcemia causes symptoms like lethargy, confusion, and renal complications.

Common Pitfalls

A frequent error is misremembering the direction of PTH and calcitonin effects. PTH increases calcium, while calcitonin decreases it. Additionally, phosphate regulation is often overlooked in calcium homeostasis.

Summary

• Calcium homeostasis involves parathyroid hormone, vitamin D, and calcitonin.
• PTH increases blood calcium via bone resorption, renal reabsorption, and vitamin D activation.
• Vitamin D boosts intestinal absorption of calcium and phosphate.
• Calcitonin reduces blood calcium by inhibiting osteoclasts.
• Hypocalcemia leads to neuromuscular excitability and tetany.
• Hypercalcemia causes lethargy and confusion.