The parathyroid hormone functions to increase the level of plasma calcium by several mechanisms.
If a patient is hypoparathyroid, it means that his/her plasma level of thyroid hormone is low. A low hypothryroid hormone means low plasma level of of calcium and this is the low plasma level of calcium that increases the excitability of peripheral nerves ( including motor nerves that supply the muscle ) and subsequently increased frequency of stimulatory signals to muscle......and eventually results in muscle cramps.