How do they work?
The parathyroid glands are monitoring calcium levels every moment of the day. When calcium levels begin to drop, the parathyroid glands produce parathyroid hormone (PTH), which pulls calcium from the bones and deposits it into the blood. When calcium reaches adequate levels, the parathyroid glands stop producing and secreting PTH. Healthy functioning parathyroid glands are constantly monitoring, and unless you have a condition that impacts the parathyroid glands, you would never know it.
What happens when calcium is too high?
Parathyroid glands stop producing and releasing PTH in order to prevent more calcium from entering the bloodstream. When calcium levels reach a point that is too high, the thyroid gland releases one of its hormones (calcitonin) which lowers the calcium levels in the blood by depositing the calcium back into the bones.
The role of Vitamin D
Vitamin D is also important to calcium regulation in the body. In the kidneys, PTH stimulates the activation of Vitamin D3 into its usable form, calcitriol. Calcitriol supports the reabsorption of calcium from the intestines, thus helping the body bring in calcium from the foods we eat.
This calcium reabsorption process is known as a feedback loop.
Click on this picture to see an outline of the process.