The Endocrine Glands - The parathyroid glands

Four small glands, each about the size of a small pea, cling to the base of the thyroid gland, two on each of the thyroid's lobes curving back of the trachea. These are the parathyroids , whose main role is to control the level of calcium—as well as other elements needed in carefully regulated amounts by the body—in the bloodstream and tissues. The parathyroids secrete two hormones: parathormone when blood calcium is too low; calcitonin when the calcium level is too high. These hormones work by controlling the interchange of calcium between bones and blood.

A symptom of defective parathyroids is tetany— a chronic or acute case of muscle spasms, which can be controlled by administration of synthetic parathyroidlike chemicals or concentrated vitamin D preparations.