Hormones

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Parathyroid hormone

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Parathyroid hormone is secreted by the parathyroid glands and is the most important regulator of calcium levels in the blood and within the bones.

Alternative names for parathyroid hormone

PTH; parathormone; parathyrin.

What is parathyroid hormone?

Parathyroid hormone is secreted from the four parathyroid glands which are small glands in the neck, located behind the thyroid gland.  Parathyroid hormone regulates calcium levels in the blood, largely by increasing the levels when they are too low.  It does this through its actions on the kidneys, bones and intestine:

  1. Kidneys - Parathyroid hormone reduces loss of calcium in urine and increases excretion of phosphate.  Parathyroid hormone also stimulates the production of active vitamin D in the kidneys.
     
  2. Bones - Parathyroid hormone stimulates the release of calcium from large calcium stores in the bones into the bloodstream.  This increases bone destruction and decreases the formation of new bone.
     
  3. Intestine - Parathyroid hormone increases calcium absorption in the intestine from food in co-operation with active vitamin D.

How is parathyroid hormone controlled?  

Parathyroid hormone is mainly controlled by the negative feedback of calcium levels in the blood to the parathyroid glands.  Low calcium levels in the blood stimulate parathyroid hormone secretion, whereas high calcium levels in the blood prevent the release of parathyroid hormone.  Small decreases in magnesium levels in the blood can also stimulate parathyroid hormone secretion, whereas large decreases can prevent it.  Parathyroid hormone may also be stimulated by increases in blood phosphate levels. 

What happens if I have too much parathyroid hormone?

Too much parathyroid hormone causes raised calcium levels in the blood (hypercalcaemia) and may be as a result of a disease called hyperparathyroidism.  There are three types - primary hyperparathyroidism and tertiary hyperparathyroidism in which the blood calcium level is high due to increased and unregulated parathyroid hormone secretion, and secondary hyperparathyroidism which is a response to low calcium levels caused by other mechanisms, eg, kidney disease.

Primary hyperparathyroidism often causes no symptoms and is frequently diagnosed by finding a high calcium concentration in a routine blood test.  Treatment is usually by surgical removal of the affected gland(s) (parathyroidectomy).  Further information on the symptoms for each condition can be found in the individual articles.

What happens if I have too little parathyroid hormone?

Too little parathyroid hormone or hypoparathyroidism, is a rare medical condition.  It can result in low levels of calcium in the blood (hypocalcaemia).  It is usually treated medically with oral calcium and vitamin D analogues.

 

Written: March 2011