parathyroid gland
Think of the parathyroid glands as four tiny smoke detectors for calcium, usually about the size of a grain of rice, tucked behind the thyroid in your neck. They do one main job: watch the calcium level in your blood and sound the alarm the instant it dips too low.
Each gland is built mostly from chief cells, which carry a calcium-sensing receptor on their surface. When blood calcium falls, the receptor senses the drop and the cells release parathyroid hormone (PTH), the body's chief calcium-raising signal. Conversely, when calcium is plentiful, the same receptor quiets the cells down. This makes the parathyroids the central thermostat of calcium control, distinct from the thyroid that surrounds them despite the shared neighborhood.
Most people have four parathyroid glands, but the number and exact location vary, and some sit far from the thyroid (for example in the chest). This anatomy matters in surgery: an overactive gland may need to be found and removed, while accidentally injuring all of them during thyroid surgery can leave a patient unable to keep calcium up, causing hypoparathyroidism.
During thyroid surgery, a surgeon carefully identifies and preserves the parathyroid glands so the patient's calcium control survives the operation.
Preserving these tiny glands prevents postoperative hypocalcemia.
Despite the similar name and shared location, the parathyroid glands are functionally unrelated to the thyroid: the thyroid sets metabolic rate, while the parathyroids govern calcium.