Which glands are primarily responsible for regulating blood calcium levels?

The parathyroid glands play a crucial role in regulating blood calcium levels by producing parathyroid hormone (PTH). When blood calcium levels drop, the parathyroid glands secrete PTH, which acts to raise calcium levels through several mechanisms. These include increasing the release of calcium from bones, enhancing the reabsorption of calcium in the kidneys, and promoting the activation of vitamin D in the kidneys, which subsequently increases calcium absorption from the intestines.

In contrast, while the thyroid glands do produce calcitonin, a hormone that helps lower blood calcium levels, their primary role is not as central as that of the parathyroid glands in the regulation of calcium. Adrenal glands are primarily involved in stress responses and the production of hormones such as cortisol and adrenaline. The hypothalamus is involved in the regulation of many bodily functions, including appetite and temperature regulation, but it does not directly regulate blood calcium levels. Thus, the parathyroid glands are the primary regulators of calcium levels in the bloodstream.