Which organ is primarily responsible for regulating calcium levels in the body?

The parathyroid gland is primarily responsible for regulating calcium levels in the body. This small gland, typically located near the thyroid gland in the neck, plays a critical role in maintaining calcium homeostasis through the secretion of parathyroid hormone (PTH).

When calcium levels in the blood are low, the parathyroid glands release PTH, which acts to increase calcium levels by stimulating the release of calcium from bones, increasing the absorption of calcium in the intestines (by promoting the activation of vitamin D), and reducing the loss of calcium through the kidneys by encouraging reabsorption. Conversely, when calcium levels are adequate or high, the secretion of PTH decreases, helping to stabilize calcium levels in the blood.

The adrenal gland, while involved in various metabolic processes and hormone production, does not play a direct role in calcium regulation. Similarly, the thyroid gland primarily regulates metabolism and has functions related to calcium metabolism through the secretion of calcitonin, but these functions are secondary to the primary role of the parathyroid hormone. The pineal gland primarily regulates sleep-wake cycles through melatonin production and does not play a role in calcium regulation.

Thus, the parathyroid gland is essential in the regulation of calcium levels in the body