How does the regulation of blood calcium concentration exemplify negative feedback and homeostasis?

When blood calcium levels are high, your body senses this and releases calcitonin from the thyroid glands. This enables calcium ions to be absorbed into bone (mineral metabolism) and prevent calcium from being absorbed from digestion. When blood calcium levels are too low, the thyroid releases parathyroid hormone which enhances the release of calcium from bone, acts on the kidneys to reabsorb calcium in the DCT, and also enhances calcium absorption from the digestive system.

The calcium levels in the bloodstream are detected by receptors in the parathyroid and thyroid glands (specifically the c-cells).

When the calcium levels are too HIGH, the thyroid c-cells release calcitonin, a hormone which lowers blood Ca2+ levels by effectively making the digestive system stop absorbing Ca2+ back into the bloodstream and then the kidneys stop reabsorbing Ca2+ from urine back into the bloodstream. Also, Ca2+ in excess in the blood starts being deposited in the bones to lower the blood Ca2+ levels to a homeostatic, normal set point.

When the calcium levels are too LOW, the parathyroid glands release PTH (Parathyroid hormone), a hormone which increases blood Ca2+ levels by making the digestive system increase Ca2+ absorption into the bloodstream and the same increase occurs in the kidneys when more Ca2+ is reabsorbed from the urine back into the bloodstream. Also, the Ca2+ stored in bones demineralizes and is released into the bloodstream to increase the blood Ca2+ levels to a homeostatic, normal set point.

The thyroid DOES NOT release PTH, nor is it specifically involved in Ca2+ regulation. Only its c-cells are responsible for helping maintain normal Ca2+ levels in the blood, and they only do so in a response to counterbalance the effects of the parathyroid glands releasing PTH.

Hope this helps.