A 45-year-old man with chronic alcoholism has tetany with corrected serum calcium of 1.6 mmol/L. Serum magnesium is 1.0 mg/dL (low). Despite a normal parathyroid response being expected, his intact PTH level is inappropriately low and rises only minimally after calcium challenge. The most likely explanation is:

• A Autonomous secretion of calcitonin from thyroid C cells
• B Accelerated degradation of PTH by activated macrophages
• C Vitamin D excess suppressing parathyroid gland function
• D Severe hypomagnesemia impairing PTH secretion and inducing end-organ PTH resistance ✓

Explanation

Magnesium is required for adenylate cyclase coupling and exocytosis in parathyroid chief cells. Severe hypomagnesemia paradoxically suppresses PTH secretion and also produces resistance to PTH at the renal tubule, so hypocalcemia cannot be corrected by calcium alone until magnesium is replaced. This is a classic examination point: give magnesium first. Vitamin A excess would raise calcium, and calcitonin has no significant role in adult calcium homeostasis.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.