What causes high calcium in blood?

Hypercalcemia, or high blood calcium, can have several causes, including:

1. Primary hyperparathyroidism: This condition occurs when the parathyroid glands, which are located near the thyroid gland, produce too much parathyroid hormone (PTH). PTH regulates calcium levels in the body, and high levels can lead to increased calcium in the blood.

2. Secondary hyperparathyroidism: This type of hyperparathyroidism develops in response to another underlying condition, such as kidney disease or vitamin D deficiency. When the kidneys are not functioning properly, they cannot effectively remove excess phosphate from the body, which can lead to increased PTH production and high blood calcium. Vitamin D deficiency can also cause secondary hyperparathyroidism, as vitamin D is essential for the absorption of calcium from the intestines.

3. Familial hypocalciuric hypercalcemia (FHH): This is a genetic condition in which the body absorbs too much calcium from food, leading to high blood calcium levels.

4. Malignant conditions: Certain types of cancer, such as lung cancer, breast cancer, and multiple myeloma, can cause hypercalcemia. Cancer cells can produce PTH-related protein (PTHrP), which acts like PTH and increases calcium levels in the blood.

5. Other causes: Less common causes of high calcium levels include certain medications (such as diuretics, lithium, and vitamin A), excessive intake of calcium supplements, Addison's disease (a disorder of the adrenal glands), and certain genetic disorders.