Four small parathyroid glands sit on the back of the thyroid. They keep blood calcium in a narrow range, which nerves, muscles and the heart need to work.
Parathyroid hormone
When calcium falls, the glands release PTH. It frees calcium from bone, makes the kidneys keep calcium and lose phosphate, and activates vitamin D.
Vitamin D
Vitamin D is made in the skin with sunlight and activated in the liver and kidneys. The active form increases calcium absorption from the gut.
Calcitonin
Cells in the thyroid release calcitonin when calcium is high. It slows bone breakdown, but its effect in adults is small.
Why calcium matters
Low calcium makes nerves too excitable, causing tingling, muscle cramps and spasm. High calcium causes weakness, constipation, kidney stones and confusion.
At the bedside
After neck or thyroid surgery, watch for low calcium: tingling around the mouth, a positive Chvostek or Trousseau sign, or a long QT on the ECG. Low albumin lowers total calcium, so check ionized calcium when the picture does not fit.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).