Four small masses of epithelial tissue are embedded in the connective tissue capsule on the posterior surface of the thyroid glands. These are parathyroid glands, and they secrete parathyroid hormone or parathormone (PTH). Parathyroid hormone is the most important regulator of blood calcium levels maintaining calcium balance in the body. The hormone is secreted in response to low blood calcium levels, and its effect is to increase those levels.

Hyper secretion of PTH is observed in primary or secondary hyperparathyroidism. 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.