How does parathyroid hormone work?

  • Blood calcium levels begin to drop
  • The parathyroid glands sense the drop and automatically release parathyroid hormone into the blood stream.
  • Parathyroid hormone stimulates the actions of vitamin D in the kidneys and intestines.
  • Bone releases more calcium into the blood.
  • Intestines absorb more dietary calcium.
  • Kidneys conserve calcium by not allowing it to be secreted in urine and it is ‘recycled’ back into the blood stream.
  • Blood calcium levels begin to rise to a ‘normal’ level and parathyroid hormone secretions automatically cease.
  • As soon as calcium levels begin to drop again the whole process starts all over again

      Below is an illustration of the PTH Feeback Loop (also called Calcium Homeostasis)

pth feedback loop

What happens when the parathyroid hormone is missing?                      

In the condition of hypoparathyroidism the essential actions of parathyroid hormone on various target organs in the body to activate vitamin D to begin to utilise calcium is missing.

In order to compensate for this ‘missing’ essential first step in the calcium feedback loop, doctors are able to prescribe an oral vitamin D analogue such as calcitriol (rocaltrol), which will act on the intestines to absorb dietary calcium and raise the levels of calcium in the blood.

Although the actions of Vitamin D analogues are life saving for patients with hypoparathyroidism it does not completely solve their calcium problems.  Unlike the automatic actions of parathyroid hormone, the vitamin D analogue is not able to either regulate or precisely maintain the level of calcium in the blood nor can it influence the kidneys to reabsorb calcium and ‘recycle’ it back into the blood stream.

This results in larger than normal amounts of calcium passing through the renal system and being excreted in urine and increased risk of renal problems or stones. It is for this reason that doctors prefer to keep their patients blood calcium levels at the lower end of the normal laboratory range and to keep their total daily dietary intake of calcium to a maximum of 2000mg if possible.  

In the absence of the automatic control of the parathyroid glands the responsibility for managing blood calcium levels is shared between the patient and their doctor. Your doctor will establish an adequate treatment regime and take regular blood tests to monitor calcium levels as well as making periodic renal health checks. The patient will make daily observations for any symptoms of low calcium and report any changes back to their doctor who will make an assessment if more blood tests are required or an adjustment in treatment.

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