The parathyroid glands are involved in the control of blood calcium levels in the body. They secrete one principal hormone, parathyroid hormone (PTH).
PTH is a peptide hormone that acts to raise blood calcium levels. It is released when calcium sensors (specialised receptor molecules) on the surface of the parathyroid glands detect low blood calcium levels in the circulation. The release of PTH is therefore inversely proportional to the calcium levels in the blood. In addition high blood magnesium levels promote the release of PTH.
PTH acts to raise blood calcium in three main ways:
1. It activates osteoclasts - these are bone cells that resorb (degrade) bone, thus releasing some of the calcium stored in bone in times of hypocalcaemia (low blood calcium) for other functions.
2. It promotes the reabsorption of calcium from the urine - PTH stimulates cells of the kidney to reabsorb calcium (and excrete phosphate) from the urine, reducing the loss of calcium in hypocalcaemia.
3. It promotes the activation of vitamin D precursors in the kidney - active vitamin D (1.25-cholecalciferol) promotes the uptake of calcium from digested food in the small intestine. This raises calcium levels when they are low.
As the calcium levels become normal, the rising calcium acts to inhibit further PTH release.
The diagram below illustrates this system.
Click to enlarge and view
The other main hormone involved in calcium regulation is calcitonin produced by the C cells of the thyroid (parafollicular cell). This is released in response to a rise in blood calcium and acts to reduce it. It does this by inhibiting the osteoclasts' resorption of bone and decreasing calcium reabsorption by the kidney. The role of calcitonin in calcium control is still not fully understood.