|
There are four parathyroid glands. They are small, light-colored lumps that stick out from the surface of the thyroid
gland. All four glands are located on the thyroid gland. They are butterfly-shaped and located inside the neck, more
specifically on both sides of the windpipe. One of the parathyroid glands most important functions is to regulate the
body's calcium and phosphorus levels. Another function of the parathyroid glands is to secrete parathyroid hormone,
which causes the release of the calcium present in bone to extracellular fluid. PTH does this by depressing the
production of osteoblasts, special cells of the body involved in the production of bone and activating osteoclasts,
other specialized cells involved in the removal of bone.
There are two major types of cells that make up parathyroid tissue:
- One of the major cells is called oxyphil cells. Their function is basically unknown.
- The second type are called chief cells. Chief cells produce parathyroid hormone.
The structure of a parathyroid gland is very different from that of a thyroid gland. The chief cells that produce
parathyroid hormone are arranged in tightly-packed nests around small blood vessels, quite unlike the thyroid cells
that produce thyroid hormones, which are arranged in spheres called the thyroid follicles.
PTH or Parathyroid Hormone is secreted from these four glands. It is released directly into the bloodstream and
travels to its target cells which are often quite far away. It then binds to a structure called a receptor, that is found
either inside or on the surface of the target cells.
Receptors bind a specific hormone and the result is a specific physiologic response, meaning a normal response of
the body.
PTH finds its major target cells in bone, kidneys, and the gastrointestinal system.
Calcitonin, a hormone produced by the thyroid gland that also regulates ECF calcium levels and serves to counteract
the calcium-producing effects of PTH.
The adult body contains as much as 1 kg of calcium. Most of this calcium is found in bone and teeth.
The four parathyroid glands secrete the parathyroid hormone (PTH). It opposes the effect of thyrocalcitonin. It does
this by removing calcium from its storage sites in bones, releasing it into the bloodstream. It also signals the kidneys
to reabsorb more of this mineral, transporting it into the blood. It also signals the small intestine to absorb more of
this mineral, transporting it from the diet into the blood.
Calcium is important for steps of body metabolism. Blood cannot clot without sufficient calcium. Skeletal muscles
require this mineral in order to contract. A deficiency of PTH can lead to tetany, muscle weakness due to lack of
available calcium in the blood.
The parathyroid glands were long thought to be part of the thyroid or to be functionally associated with it. We now
know that their close proximity to the thyroid is misleading: both developmentally and functionally, they are totally
distinct from the thyroid.
The parathyroid hormone, called parathormone, regulates the calcium-phosphate balance between the blood and
other tissues. Production of this hormone is directly controlled by the calcium concentration of the extracellular fluid
bathing the cells of these glands. Parathormne exerts at least the following five effects:
- it increases gastrointestinal absorption of calcium by stimulating the active transport
system and moves calcium from the gut lumen into the blood;
- it increases the movement of calcium and phosphate from bone into extracellular fluid.
This is accomplished by stimulating osteoclasts to break down bone structure,
thus liberating calcium phosphate into the blood. In this way, the store of calcium contained in bone is tapped;
- it increases re-absorption of calcium by the renal tubules, thereby decreasing urinary calcium excretion;
- it reduces the re-absorption of phosphate by the renal tubules
- it stimulates the synthesisof 1,25-dihydrixycholecalciferol by the kidney.
The first three effects result in a higher extracellular calcium concentration. The adaptive value of the fourth is to
prevent the formation of kidney stones.
If parathyroid glands are removed accidentally during surgery on the thyroid, there would be a rise in the phosphate
concentration in the blood. There would also be a drop in the calcium concentration as more calcium is excreted by
the kidneys and intestines, and more incorporated into the bone. This can produce serious disturbances, particularly
in the muscles and nerves, which use calcium ions for normal functioning. Over activity of the parathyroid glands,
which can result from a tumor on the glands, produces a weakening of the bones. This is a condition that makes them
much more vulnerable to fracturing because of excessive withdrawal of calcium from the bones.
|