Hello and welcome to the Zero to Finals podcast.

My name is Tom and in this episode I'm going to be talking to you about the parathyroid axis and calcium regulation.

And you can find written notes on this topic at zero to finals.com

slash parathyroid axis or in the endocrinology section of the second edition of the

zero to finals medicine book. And you can find flashcards and questions to train your knowledge

on this content

and help you remember the information for longer at members.0 tofinals.com.

So let's get straight into it.

There are four small parathyroid glands situated in the four corners of the thyroid gland.

The parathyroid glands produce parathyroid hormone, or PTH, in response to a lower calcium

in the blood.

Parathyroid hormone acts to increase the serum calcium, or the calcium in the blood,

by targeting three organs. The bones, increasing calcium absorption from the bones,

the kidneys increasing calcium reabsorption from the urine, and the intestines by increasing vitamin D activity

which increases calcium absorption from food. The serum calcium level is ideally maintained

between 2.2 and 2.6 millimoles per liter.

Let's go through the parathyroid hormone physiology.

Inside the parathyroid glands are the chief cells, which produce parathyroid hormone, and

store it inside secretory granules.

The secretory granules can fuse with the cell membrane of the chief cells and release

parathyroid hormone into the circulation, and this is called exocytosis.

The chief cells have calcium sensing receptors, or C-A-S-R, on their cell surface, which detect calcium.

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