In CKD, what is the consequence of phosphate retention on free calcium and PTH?

In CKD, what is the consequence of phosphate retention on free calcium and PTH?

• A. Free calcium falls because phosphate binds calcium; this stimulates PTH secretion.
• B. Free calcium rises due to phosphate buffering.
• C. Free calcium remains unchanged; PTH is suppressed.
• D. Free calcium falls but PTH falls as well.

Answer: (C)

In CKD, phosphate retention raises serum phosphate. Phosphate binds calcium, reducing the amount of free (ionized) calcium in the blood. This drop in ionized calcium is detected by the parathyroid glands, which respond by increasing parathyroid hormone (PTH) secretion. PTH then acts to raise calcium levels, but in CKD the kidneys’ ability to fully respond is impaired and vitamin D activation is reduced, so the hypocalcemia can persist and drive ongoing secondary hyperparathyroidism. So the consequence is a fall in free calcium with stimulation of PTH secretion.

In CKD, phosphate retention raises serum phosphate. Phosphate binds calcium, reducing the amount of free (ionized) calcium in the blood. This drop in ionized calcium is detected by the parathyroid glands, which respond by increasing parathyroid hormone (PTH) secretion. PTH then acts to raise calcium levels, but in CKD the kidneys’ ability to fully respond is impaired and vitamin D activation is reduced, so the hypocalcemia can persist and drive ongoing secondary hyperparathyroidism. So the consequence is a fall in free calcium with stimulation of PTH secretion.