Four parathyroid glands lie adjacent to the thyroid gland. These are small glands, measuring 4-8 mm in size. They share blood supply with the thyroid gland, and their function can be impaired during thyroid surgery. Parathyroid glands contain cells that produce parathyroid hormone (PTH). PTH is a critical hormone that plays a vital role in maintaining the proper level of calcium in the blood and tissues. Calcium is required for normal cell function. Parathyroid hormone causes release of calcium from bone stores, increased absorption of calcium from the gut, and reabsorption of calcium from the kidneys. The parathyroid glands can develop adenomas that produce excess PTH, resulting in hyperparathyroidism.