Four small, yellowish-brown, pea-like masses of epithelial
tissue that are embedded in the connective
tissue capsule on the posterior surface of the thyroid
glands. The parathyroid glands are endocrine
glands which secrete parathyroid
hormone or parathormone. Parathyroid hormone is the most important regulator
of blood calcium levels. The hormone is
secreted in response to low blood calcium levels, and its effect is to increase
|The hormone produced by the parathyroid glands (orange) abbreviated to PTH, has the important function of regulating the level of calcium in the blood. The mineral itself controls secretion of PTH by means of a feedback mechanism. A low blood-calcium level (A) triggers secretion of PTH. This in turn mobilizes calcium from the bones (blue) and aids its absorption from the intestines (green) and reabsorption from the kidneys (pink) to boost the amount that is circulating in the blood. With a high blood-calcium level (B) the sequence is reversed. Production of the parathyroid hormone is inhibited and consequently more calcium is laid down in the bones of the body.
Hypoparathyroidism, or insufficient
secretion of parathyroid hormone, leads to increased nerve excitability.
The low blood calcium levels trigger spontaneous and continuous nerve
impulses, which then stimulate muscle contraction.
Hyperparathyroidism is the excessive secretion of parathyroid hormone, usually due to a small
tumor in one of the parathyroid glands. It results in hypercalcemia.
Parathyroidectomy is the surgical removal of
the parathyroid glands, usually as part of the treatment of hyperparathyroidism.