These are several minute glandular masses embedded in the lower edge of the thyroid gland. They produce Parathyroid Hormone (PTH), part of the calcium-phosphorus control system. Calcium levels in the blood MUST be within a narrow band of safety. If free calcium drops too low, PTH acts on the kidneys and blocks calcium loss in urine, amplifies calcium absorption into the portal blood (from food and from submucosal storage) and stimulates release of calcium from bone storage. When levels are back up, the hormone backs off. Oddly enough, the thyroid gland secretes its virtual antagonist, calcitonin, which, when calcium levels are too high, stimulates the urine excretion, bone retention and digestive resistance to calcium, and when the blood levels drop, recedes. The body finds calcium levels to be so critical that it has in place TWO separate, mutually antagonistic negative feedback systems,,,like a binary star system. (Be thankful I didn't bring in the calcium maintenance of minerocortical steroid hormones or vasopressin)