Which hormone primarily functions to increase the calcium level in the blood?

The hormone that primarily functions to increase calcium levels in the blood is parathyroid hormone (PTH). PTH is secreted by the parathyroid glands and plays a crucial role in maintaining calcium homeostasis in the body.

When calcium levels in the blood decrease below a certain threshold, PTH is released into the bloodstream. This hormone acts on various target organs to elevate calcium levels. It stimulates osteoclasts in the bones, which are cells that break down bone tissue and release calcium into the bloodstream. PTH also enhances the reabsorption of calcium in the kidneys, reducing the amount lost in urine, and it increases the production of active vitamin D in the kidneys, which promotes the absorption of calcium from the intestines.

In contrast, calcitonin, which is secreted by the thyroid gland, has the opposite effect by lowering blood calcium levels. Insulin is primarily associated with glucose metabolism, while adrenaline is involved in the 'fight or flight' response and does not directly regulate calcium levels in the blood. Thus, parathyroid hormone's role in increasing blood calcium makes it the correct answer.