Parathyroid Gland Operation

What are parathyroid glands?

Parathyroid glands are 4 (usually) small pea-sized glands that are located next to the thyroid gland. Most people have 4 parathyroid glands, but some-very rarely-may have an extra parathyroid gland, or even 2, or even 3. The usual shape of the parathyroid glands is oval or bean-like and their color tends to be yellow-brown. They measure 5-6 x 2-3 x 1-2 mm in size and weigh approximately 40 to 50 mg each. Histologically, parathyroid glands are composed of chief cells and oxyphil cells arranged in trabeculae, within a stroma composed primarily of adipose cells. The role of chief cells is to produce parathyroid hormone (PTH) through which they are able to maintain calcium homeostasis. Oxyphil and water-clear cells retain the ability to secrete PTH, their functional significance is unclear.

Where are they located?

All mammals have four parathyroid glands situated in proximity to the thyroid gland. In humans the superior pair are the more constant in position on the posterior border of the thyroid gland midway between the lower and upper poles of the thyroid gland within a 1cm radius of the junction of the recurrent laryngeal nerve and inferior thyroid artery. The inferior pair lie in a more variable position, usually within the thyrothymic tract, a condensation of fascia between the thyroid gland and the thymus. Supernumerary glands are present in up to 13% of patients.

The superior glands are usually positioned dorsal to the recurrent laryngeal nerve (RLN) at the level of the cricoid cartilage and are derived from the fourth branchial pouch, whereas the inferior parathyroid glands are located ventral to the nerve and are derived from the third branchial pouch. The primary blood supply to the glands originates from the inferior thyroid artery while the superior thyroid artery or the thyroid ima artery may also rarely supply these glands.

What do the parathyroid glands do?

The parathyroid glands produce a hormone called parathyroid hormone (PTH). This hormone aims to keep the calcium in the blood at steady levels at all costs (sometimes even at the expense of other organs such as bones and kidneys).

The superior glands are usually positioned dorsal to the recurrent laryngeal nerve (RLN) at the level of the cricoid cartilage and are derived from the fourth branchial pouch, whereas the inferior parathyroid glands are located ventral to the nerve and are derived from the third branchial pouch. The primary blood supply to the glands originates from the inferior thyroid artery while the superior thyroid artery or the thyroid ima artery may also rarely supply these glands.

Role of calcium

Calcium plays a key role in numerous metabolic functions.

It is used by our muscles in order to function (the heart is a muscle), it is used in the brain as a neurotransmitter in between the brain cells and of course in our bones to make them stronger. It is the most abundant cation in humans and the extracellular calcium levels are 10,000-fold higher than intracellular levels. Extracellular calcium is important for excitation–contraction coupling in muscle tissues, synaptic transmission in the nervous system, coagulation, and secretion of other hormones while intracellular calcium is an important second messenger regulating cell division, motility, membrane trafficking, and secretion.

Calcium is absorbed from the small intestine in its inorganic form. Extracellular calcium (900 mg) accounts for only 1% of the body’s calcium stores, the majority of which is found in the bones. Approximately 50% of the serum calcium is in the ionized form, which represents the active component. The remainder is bound to albumin (40%) and organic anions such as phosphate and citrate (10%). The total serum calcium levels ranges from 2.2 to 2.6 mmol/L.

How do parathyroid glands control calcium?

The parathyroid glands have a receptor in their surface that constantly measures how much calcium we have in our blood. When the calcium in the blood becomes low, the parathyroid glands will understand that and will produce more PTH in order to counter-balance that. If the blood calcium normalises, then the production of PTH will go down to normal levels.

 

How does PTH act?

The PTH acts in the bones by promoting the destruction of the bone and as such the calcium is released inside the blood. It also acts in the kidneys by decreasing the calcium excretion (otherwise the calcium would be lost in the urine).
The PTH works together with the Vitamin D to achieve the goal of keeping the calcium in the blood at steady levels. Vitamin D works by reabsorbing the calcium in the gut and putting it in the blood.

PTH synthesis is regulated by a gene in chromosome 11 and begins with the secretion of a precursor hormone, the preproparathyroid hormone. This precursor is cleaved sequentially to the proparathyroid form and finally to the 84-amino acid parathyroid hormone. Parathyroid hormone has a half-life of approximately 4 minutes, is metabolized by the liver and excreted by the kidneys.

Normal conditions

When the parathyroid glands work normally, there is a perfect balance in our body and the effect of PTH on the bones and the kidneys is not enough to cause any damage to these organs.

History of the discovery of the parathyroid glands

It was Sir Richard Owen (1804–1892), Hunterian Professor and Conservator of the Museum in the Royal College of Surgeons of England, who was the first to describe the parathyroid glands, in 1852. In May 1834 the Zoological Society of London purchased its first Great Indian Rhinoceros (Rhinoceros unicornis). When the animal died on the evening of 19 November 1849, its carcass was offered for an autopsy to Sir Richard Owen. He described in detail a glandular, small, yellow, compact body which was attached to the thyroid gland, where the veins emerged. However, he was not aware of the physiology and pathophysiology of these glands. He published his work as the third article in the fourth volume of the Society’s Transactions, which volume, covering the period January 1851 to September 1862, bears the terminal date of 1862. For in those early days of the Zoological Society’s publications, the individual papers comprising any particular volume of its Transactions appeared separately, under dates anterior to the terminal volume date. Thus Owen’s rhinocerotine memoir was published, not in 1862 as commonly supposed, but full ten years earlier, on March 1852. It was not until A.J.E. Cave, successor to Owen as Professor of Anatomy at the RCS, discovered that Owen’s paper was published in 1852—not 1862 as originally thought. In honor of this discovery, A.J.E. Cave in 1953 gave these glands the term ‘glands of Owen’.

Although Owen’s description is the earliest reference to the existence of the parathyroid glands, it is the Swedish-born Ivar Viktor Sandström (1852–1889) who is credited with discovering and naming the glands. He was a medical student employed as a recorder in the department of anatomy in the University of Uppsala when he came across structures, in a dog’s neck, that were of interest to him. In 1880, he stated: ‘I encountered on the thyroid of a dog a small hardly hemp-seed sized structure which was included in the same capsule as the thyroid but distinguished itself from it by a brighter colour. A superficial examination revealed an organ of a structure entirely different from that of the thyroid and particularly amply vascularised, because of which I considered it probable that here a vascular gland had been encountered, analogous to the carotid glands’. Having then identified the organ in other animals including cat, rabbit, ox and horse, Sandström proceeded to human anatomy and, to his astonishment, in the first individual examined, ‘found on both sides of the inferior border of the thyroid an organ of the size of a small pea which judging from its exterior, did not appear to be a lymph gland, or an accessory thyroid gland and which upon histological examination showed a rather peculiar structure’. He named the new structures glandulae parathyroidae. Sandström was not aware of Owen’s description and because his report was rejected by German editors, it was published in 1880 in a local Swedish journal, the ‘Ups&r Liikarefiirenings Fiirhundlinger’ and went barely noticed for several years.

Source: I.Christakis et al. The History of the Parathyroid Glands: A Brief Review.