Although the thyroid and parathyroid glands are physically located close to each other and are both components of the endocrine system, their functions are distinct. Typically, there are four parathyroid glands, situated near the thyroid gland. Each parathyroid gland is roughly the size of a grain of rice, measuring approximately 3-5 millimeters in diameter and weighing 30-60 milligrams. These glands secrete parathyroid hormone (PTH), which plays a vital role in regulating calcium levels within the bloodstream. Other parts of the body, particularly the bones, kidneys, and small intestine, respond to PTH by promoting an increase in blood calcium levels.