What explains hypocalcemia after thyroid surgery in terms of PTH and calcium/phosphate interaction?

• A. Loss/dysfunction of the parathyroids reduces PTH, leading to decreased bone resorption and renal calcium reabsorption, causing hypocalcemia and hyperphosphatemia.
• B. Increased PTH leads to calcium leaching from bone causing hypercalcemia.
• C. Thyroidectomy increases calcitonin causing calcium loss.
• D. PTH remains normal; phosphate normal.

Answer: (A)

When the parathyroid glands are damaged or removed during thyroid surgery, parathyroid hormone (PTH) drops, and that has a big impact on calcium and phosphate balance. PTH keeps serum calcium up by three means: it promotes calcium release from bone (bone resorption), it increases calcium reabsorption in the kidneys, and it raises active vitamin D production to boost gut calcium absorption. It also lowers serum phosphate by increasing its excretion in the kidneys.

With low PTH, you get less calcium released from bone and less calcium reabsorbed in the kidneys, so calcium falls in the blood. At the same time, the kidneys reabsorb more phosphate when PTH is low, so phosphate rises, leading to hyperphosphatemia. This combination explains hypocalcemia after thyroid surgery.

The other ideas don’t fit as well: increased PTH would raise calcium (hypercalcemia), and calcitonin would tend to lower calcium but isn’t driven by thyroidectomy in this scenario. PTH remaining normal with normal phosphate wouldn’t account for the postoperative hypocalcemia and hyperphosphatemia.