Parathyroid glands are active parts of the endocrine system. The parathyroid hormone has a relatively short life span in the blood stream, its levels falling by 50 per cent every four minutes. The four tiny parathyroid glands that are situated at the back of the thyroid gland produce the parathyroid hormone (PTH), the major regulator of calcium levels in blood.
The correct balance of calcium is essential for many functions, including muscle contractions and the transmission of nerve impulses. Thus, it needs to be controlled precisely. When blood calcium levels fall too low, PTH stimulates the release of stored calcium from bone into the blood and reduces calcium loss from the kidneys into urine. All in all, indirectly it is related to increasing the absorption of calcium from ingested food in the small intestine. Here, vitamin D is required in order to let the intestine absorb calcium. But, the ingested form is inactive. Thus, PTH stimulates the kidneys to convert vitamin D from its precursor form into its active form, calcitriol.
Effects of parathyroid hormone:
This hormone acts on the bone, kidneys, and although indirectly, it also acts on the small intestine to increase the amount of calcium in the blood.