What is true regarding the use of PTH in calcium metabolism disorders?

The correct answer highlights the role of parathyroid hormone (PTH) in distinguishing between primary and secondary causes of hypocalcemia. In cases of hypocalcemia, measuring PTH levels provides critical information.

When calcium levels drop, the parathyroid glands release PTH to help increase serum calcium by promoting calcium release from bones, increasing renal reabsorption of calcium, and stimulating the production of active vitamin D, which increases gastrointestinal absorption of calcium. In the context of primary hypoparathyroidism, PTH levels would typically be low due to the lack of parathyroid function. In contrast, with secondary hypoparathyroidism—often caused by vitamin D deficiency or chronic kidney disease—the body usually responds to low calcium by increasing PTH secretion. Thus, if PTH levels are elevated alongside hypocalcemia, one could infer that the patient may be experiencing secondary causes of hypocalcemia.

This distinction is essential for clinicians to determine the appropriate treatment course, making PTH measurement a significant tool in calcium metabolism disorders.