Identify the order of calcium homeostasis write the number on the blank before the statement

Alternative names for parathyroid hormone

PTH; parathormone; parathyrin

What is parathyroid hormone?

The parathyroid glands are located in the neck, just behind the butterfly-shaped thyroid gland.

Parathyroid hormone is secreted from 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. 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.
    
2. Kidneys – parathyroid hormone reduces loss of calcium in urine. Parathyroid hormone also stimulates the production of active vitamin D in the kidneys.
    
3. Intestine – parathyroid hormone indirectly increases calcium absorption from food in the intestine, via its effects on vitamin D metabolism.

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. 

What happens if I have too much parathyroid hormone?

A primary problem in the parathyroid glands, producing too much parathyroid hormone causes raised calcium levels in the blood (hypercalcaemia) and this is referred to as primary hyperparathyroidism. There is a similar but much rarer condition called tertiary hyperparathyroidism that causes hypercalcaemia due to excess parathyroid hormone production on the back drop of all four glands being overactive. Secondary hyperparathyroidism occurs in response to low blood calcium levels and is caused by other mechanisms, for example, kidney disease and vitamin D deficiency.

Mild primary hyperparathyroidism often causes few if any symptoms and is frequently diagnosed by finding a high calcium concentration on a routine blood test. Treatment may be 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 but the availability of parathyroid hormone replacement therapy may change the approach to treatment for some patients.

Last reviewed: Feb 2018

The body maintains very tight control over the calcium circulating in the blood at any given time. The equilibrium is maintained by an elegant interplay of calcium absorbed from the intestines, movement of calcium into and out of the bones, and the kidney’s reclamation and excretion of calcium into the urine.

If the serum calcium level falls, the parathyroid glands release PTH into the blood and this signals cells in bone (osteoclasts) to release calcium from the bone surfaces. PTH also signals the kidney to reclaim more calcium before it is excreted in the urine and also stimulates synthesis of the active form of vitamin D.

There are at least three hormones intimately involved in the regulation of the level of calcium in the blood: parathyroid hormone (PTH), calcitonin and calcitriol (1, 25 dihydroxyvitamin D, the active form of vitamin D). PTH comes from the parathyroid glands located behind the thyroid gland in the lower part of the neck and calcitonin comes from cells in the thyroid gland, both of which monitor and maintain calcium levels in the blood. The active form of vitamin D is synthesized in the kidney under the control of PTH.

Special cells that reside in the thyroid gland along with thyroid hormone containing cells release another hormone, calcitonin, into the blood. Calcitonin signals osteoclasts to slow down removal of calcium from bone; this action tends to lower levels of blood calcium. Conversely, shutting off calcitonin allows osteoclasts to get back in business to release needed calcium from bone.

The PTH system provides long-term, day-to-day regulation of calcium levels by many hormones working in concert. This hormonal “feedback loop” is governed by the parathyroid glands and the calcitonin-secreting cells of the thyroid gland by their constant monitoring of the blood calcium levels. The body also has a minute to minute regulation of calcium levels from osteocytes in bone—these cells can instantly release needed calcium or instantly stop releasing calcium depending on immediate needs (too little or too much calcium coming into the bloodstream).

Reviewed: 5/14/19

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