What is a common cause of PTH-independent hypercalcemia?

• A. Lithium usage
• B. Familial hypocalciuric hypercalcemia
• C. Acute kidney injury
• D. Primary hyperparathyroidism

Answer: (A)

PTH-independent hypercalcemia can occur due to several conditions where parathyroid hormone (PTH) levels are not responsible for the elevated calcium levels. Lithium usage is known to affect calcium metabolism; it can interfere with the parathyroid gland's ability to regulate calcium and may lead to increased levels of serum calcium without an increase in PTH.

In contrast, familial hypocalciuric hypercalcemia is a genetic condition where altered calcium sensing leads to increased calcium levels but is characterized by high levels of PTH. Acute kidney injury can also lead to changes in calcium levels but primarily through complex renal handling of electrolytes and is not classified as PTH-independent hypercalcemia. Primary hyperparathyroidism, by definition, involves elevated levels of PTH leading to increased calcium levels, thus being dependent on PTH.

Therefore, lithium usage stands out as a substance that directly contributes to PTH-independent hypercalcemia by disrupting normal calcium homeostasis.