Parathyroid Function

The parathyroid glands are located in the neck immediately adjacent, on or even rarely within the thyroid.  Each parathyroid gland is approximately 3x4x5 millimeters and typically weighs 25-40 milligrams. Parathyroid glands produce the peptide hormone PTH, which plays a crucial role in calcium homeostasis. There are typically four parathyroid glands (a superior and inferior gland on each side of the neck), but in rare circumstances patients may have fewer or more glands.  The superior glands are located posterior to the thyroid lobes usually superior to the inferior thyroid artery and posterior to the recurrent laryngeal nerve.  The inferior glands are typically found adjacent to the inferior poles of the thyroid, below the inferior thyroid artery and anterior to the recurrent laryngeal nerve.  While 90% of these glands are found in the normal or “expected” location, ectopic glands may be seen.  The parathyroid glands arise from the 3rd (inferior) and 4th (superior) pharyngeal pouches and follow distinct paths as they descend into their normal locations.  A thorough knowledge of the anatomy and embryology of the parathyroid glands is crucial to the parathyroid surgeon and will guide the search for ectopic glands when aberrant anatomy is encountered.   For example, when a missing inferior gland is noted, the thymus is a common ectopic location that should be investigated, as this structure also arises from the 3rd pouch. In addition to the thymus, ectopic parathyroids may be found high in the neck (undescended), in the chest, embedded in the thyroid gland or in the sheath surrounding the carotid artery and jugular vein. 

The parathyroid glands play a crucial role in calcium homeostasis via the production of parathyroid hormone (PTH).  Serum calcium binding to the calcium sensing receptor on the parathyroid gland is the major input regulating the production and secretion of PTH, although vitamin D and magnesium have some effect as well.  Because it functions as a negative feedback loop, the thermostat analogy is a useful one to use when discussing the parathyroid glands with patients.  Low calcium (low temperature), reflected in decreased activation of the calcium sensing receptor on the parathyroid glands (thermostat) results in increased production and secretion of PTH.  This PTH, in turn, acts to raise serum calcium (temperature) via its action on several end organs (furnace). 

1. The kidneys

         a) Increased calcium reabsorption

         b) Decreased phosphate reabsorption

         c) Increased conversion of vitamin D to its final form – 1,25(OH) vitamin D

2. Bones (increased resorption)

3. Intestines (increased absorption largely due to increased vitamin D in its final form)

Once the calcium level is increased, binding of calcium to the calcium sensing receptor reduces the production and secretion of PTH returning these levels to normal. 

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