Although this book is dedicated to diseases of the thyroid gland, it's important to know a little bit about the parathyroid glands. Parathyroid glands are four tiny glands — some people may have more or fewer — located behind the thyroid gland. Unlike the thyroid gland, which is made up of follicles, the parathyroid glands are composed of distinct, densely packed cells. These glands produce parathyroid hormone (PTH), which regulates the metabolism of calcium and phosphorous. They also play a role in maintaining healthy bones.

When blood-calcium levels drop below a certain point, calcium-sensing receptors in the parathyroid gland respond by releasing PTH into the blood. PTH then stimulates osteoclasts to break down bone and release calcium into the blood. By keeping the calcium levels in our body within a narrow range, our nervous and muscular systems are able to function properly.

The parathyroid glands, however, are separate and distinct from the thyroid gland. One can be healthy while the other is diseased. But the health of the parathyroid glands can be affected during thyroid surgery, which is why it's critical that the surgeon not harm the parathyroid glands during a thyroidectomy, removal of the thyroid gland.