**Question:** A patient undergoes thyroid surgery, following which he develops perioral tingling. Blood calcium is 8.9 meq/L. Next step is:

A. Thyroid function test
B. Calcium supplementation
C. Serum magnesium estimation
D. Serum phosphorus estimation

**Core Concept:** Hypocalcemia is a condition characterized by low blood calcium levels. In this scenario, the patient develops perioral tingling post-thyroid surgery, which is a manifestation of hypocalcemia. Hypocalcemia can lead to neuromuscular excitability and muscle cramps, among other symptoms.

**Why the Correct Answer is Right:** The correct answer is C. Serum magnesium estimation, as hypocalcemia can be caused by hypomagnesemia and hypomagnesemia can lead to neuromuscular excitability and symptoms similar to hypocalcemia. The other options are not directly related to the management of hypocalcemia:

- A: Thyroid function test may be relevant to identify the cause of hypocalcemia (e.g., hypoparathyroidism, primary hyperparathyroidism), but it does not directly address the management of hypocalcemia.
- B: Calcium supplementation is a treatment for hypocalcemia, but it is not the immediate next step after identifying the cause. Magnesium estimation is necessary to rule out hypomagnesemia as a potential cause of hypocalcemia.
- D: Serum phosphorus estimation is not directly related to the management of hypocalcemia. Phosphorus does not affect calcium homeostasis directly, unlike magnesium.

**Why Each Wrong Option is Incorrect:**

- A: Calcium supplementation is relevant when the cause of hypocalcemia is known, not as the initial step after identifying hypocalcemia symptoms.
- B: While calcium supplementation is a treatment for hypocalcemia, it is not the correct next step after identifying the cause. Magnesium estimation is necessary to rule out hypomagnesemia as a potential cause of hypocalcemia.
- D: Serum phosphorus estimation is not directly related to the management of hypocalcemia. Hypophosphatemia is not a common cause of hypocalcemia and does not directly affect calcium homeostasis.

**Clinical Pearl:** Hypomagnesemia can coexist with hypocalcemia, as magnesium is a cofactor for calcium-regulating enzymes like parathyroid hormone (PTH) and calcitonin. Hypomagnesemia can impair PTH secretion and activity, leading to hypocalcemia. Therefore, correcting magnesium levels is essential in managing hypocalcemia, as it affects the regulation of calcium homeostasis.