Parathyroid glands are small endocrine glands located in the neck, adjacent to and behind the thyroid gland. Most people have four parathyroid glands: 2 located on either side of the thyroid gland. On occasion, people have extra parathyroid glands, or glands that are in different locations than usual. However, most people with these variations do not suffer from any problems. The diagram shows the location of the parathyroid glands in yellow, with the normal thyroid gland in pink. The circular structure running through the center of the image is the wind pipe, or trachea. The voice box, or vocal cords, sit near the top of the trachea and the carotid arteries, which supply blood and oxygen to your brain and rest of your head, are shown in red. Important to note is that the parathyroid and thyroid glands, although named similarly, are not related in function.
Now that we have an overview of the location of the parathyroid glands, let us discuss their function. In a normal individual, the parathyroid gland secretes parathyroid hormone (PTH for short) which helps to regulate the body's calcium levels. When the level of calcium in the blood is low, the parathyroid glands secrete extra PTH into the blood. The increase in PTH levels then travels to your intestines where it causes your body to absorb more calcium from your food. It also travels to the kidneys where it results in less calcium being excreted into the urine, and to your bones where it activates cells that break down bone to release calcium back into the blood. All of these mechanisms help to restore appropriate levels of calcium in the blood. As you can imagine, the opposite also holds true. If the calcium levels in the blood are too high, the parathyroid glands will produce less PTH and thus decrease the amount that is absorbed from the intestines and bones as well as increase the amount that is excreted in the urine and flushed out of the body.
By now, you must be wondering why calcium is so important to the body and what effects it can have if the levels become too high or too low. Calcium is important for many bodily processes, particularly for our nerves, muscles, and bones. Let's discuss each of these:
Nerves: Signals are conducted from your brain out to your body, and from your body back to your brain via nerves. Amazingly, these signals are conducted using electricity. Calcium is one of the most important molecules used in the body to conduct this electricity down the nerves. Therefore, an imbalance of calcium levels can lead to various nerve-related problems such as weakness, tiredness, numbness, and tingling.
Muscles: Similar to nerves, muscles use calcium as a signal of when to move, or contract. Therefore, changes in calcium levels can cause muscle weakness as well as cramping.
Bones: Most people are aware that calcium is necessary for building strong bones. However, what most people don't know is that bones also serve as a reservoir for calcium, storing and releasing it when needed, much like a dam for a river. Because the blood level of calcium is so important for survival ,the body will continue to pull calcium out of bones to maintain blood levels. Chronically low calcium levels can lead to bone weakness and fractures, but this phenomenon is usually seen in the elderly after decades of low levels.
So as you can see, calcium is one of the most closely monitored elements in our bodies, and is the only one to have a gland dedicated entirely to its regulation: the parathyroid gland. When calcium levels are low, the parathyroid glands release parathyroid hormone, which circulates through the blood and activate absorption of calcium from the intestines and bone, as well as decrease the excretion of calcium from the kidneys. This entire process is tightly regulated to keep calcium levels within a very tight range, usually between 9-10.5 milligrams of calcium per deciliter of blood (mg/dL). To put that in perspective, that is approximately the same concentration as putting 1 pinch of table salt into one gallon (4 liters) of water, and the average human body only contains about 1.25 gallons (or 5.5 liters) of total blood (may rephrase this)! This entire volume of blood is circulated through the entire body In the next section, we will discuss one of the most common diseases of the parathyroid gland: hyperparathyroidism, or the excretion of too much parathyroid hormone.