Lithium's Role in Causing Hypercalcemia

Lithium is known to cause hypercalcemia, a condition characterized by abnormally high calcium levels in the blood. This effect is primarily due to lithium's impact on the parathyroid glands.

Mechanism of Lithium-Induced Hypercalcemia

• Parathyroid Hormone (PTH) Regulation: Lithium can interfere with the normal regulation of parathyroid hormone (PTH) secretion and action.
• Increased PTH Levels: It may increase PTH levels or enhance the sensitivity of bone and kidney cells to PTH, leading to increased calcium reabsorption and release from bones.
• Impaired Calcium Sensing: Lithium can affect the calcium-sensing receptors (CaSR) on the parathyroid glands, potentially reducing their ability to detect high calcium levels and thus suppressing PTH secretion appropriately. This disruption can result in sustained or elevated PTH activity, contributing to hypercalcemia.

Analysis of Other Options

• Phenytoin: While phenytoin can affect calcium metabolism, it is not typically associated with causing hypercalcemia. In some cases, it might be linked to hypocalcemia.
• Heparin: Heparin is an anticoagulant and does not directly cause hypercalcemia.
• Furosemide: Furosemide is a loop diuretic that actually promotes the excretion of calcium in the urine, often leading to hypocalcemia (low calcium levels), not hypercalcemia.

Therefore, among the given options, lithium is the drug most commonly associated with causing hypercalcemia.