Which hormone is primarily responsible for controlling blood calcium levels?

The hormone that primarily regulates blood calcium levels is parathyroid hormone, which is secreted by the parathyroid glands in response to low blood calcium levels. This hormone increases calcium levels in the bloodstream by stimulating several processes: it promotes the release of calcium from bones, increases intestinal absorption of calcium by activating vitamin D, and reduces calcium loss through the kidneys.

Calcitonin, while mentioned in the options, plays a role in calcium regulation but serves a different purpose. It is produced by the parafollicular cells of the thyroid gland and primarily functions to lower blood calcium levels when they are elevated. It does so by inhibiting osteoclast activity (the cells that break down bone) and promoting calcium deposition in bones.

Epinephrine, often associated with fight-or-flight responses, does not directly regulate calcium levels. It mainly influences metabolism and cardiovascular responses. Similarly, thyroid hormone affects metabolism and has various systemic effects but does not have a primary role in calcium regulation.

Thus, parathyroid hormone stands as the key regulator of calcium levels in the body, making it the correct answer in this context.