What is a consequence of severe hypomagnesemia on PTH secretion and calcium homeostasis?

• A. Increased PTH secretion with hypercalcemia
• B. Impaired PTH secretion and action leading to hypocalcemia
• C. Normal PTH with normal calcium
• D. PTH resistance with hypercalcemia

Answer: (B)

Severe low magnesium directly disrupts the parathyroid system in two ways: it impairs the release of parathyroid hormone and it also reduces the body's responsiveness to that hormone. Magnesium is a necessary cofactor for the cellular machinery that secretes PTH, so when magnesium is critically low, PTH secretion drops or becomes inappropriately normal despite low calcium. At the same time, magnesium deficiency causes resistance to PTH at target organs like bone and kidney, blunting PTH’s ability to promote calcium release from bone, calcium reabsorption in the kidney, and activation of vitamin D. The combined effect is lower serum calcium (hypocalcemia) with insufficient PTH activity. In practice, restoring magnesium is essential to correcting the hypocalcemia, as calcium administration alone may not fully resolve it until magnesium levels are back in the normal range.