Corpus: Parathyroid gland

The parathyroid glands in humans are four small endocrine organs that play a crucial role in regulating calcium balance.

The four parathyroid glands are typically located near the upper and lower poles of the thyroid gland. Therefore, they are classified as:

• Upper parathyroid glands: Superior parathyroid glands
• Lower parathyroid glands: Inferior parathyroid glands

However, the exact location of the parathyroid glands can vary significantly due to embryological development. It is not uncommon for the parathyroid glands to be found in atypical locations, such as behind the sternum (retrosternally).

Histologically, the parathyroid gland is divided into three cell types:

• Principal cells: These are polygonal cells with a round nucleus, measuring about 6 to 8 μm in diameter. They can be further classified into light-colored, glycogen-rich cells and dark-colored, glycogen-poor cells. Their primary function is to synthesize parathyroid hormone (PTH).
• Oxyphilic cells: These cells are lighter in color compared to principal cells due to their high mitochondrial content, making them acidophilic. They are relatively large, measuring 12 to 20 μm, with small nuclei. The number of oxyphilic cells increases with age in humans. As of 2023, their exact function remains unknown.
• Univacuolar fat cells: These cells are also present within the parathyroid gland, although the text does not specify their detailed characteristics or functions.

The parathyroid glands originate from the endoderm. The precursors of the upper parathyroid glands develop from the dorsal bud of the 4th pharyngeal pouch, while the precursors of the lower parathyroid glands develop from the dorsal bud of the 3rd pharyngeal pouch. These precursors migrate caudally from their respective pharyngeal pouches during development.

The parathyroid glands produce parathyroid hormone (PTH), which is the most crucial hormone for calcium homeostasis. The secretion rate of PTH as a function of calcium levels follows an S-curve. The most significant increase in secretion occurs around the set point of the control loop, giving the regulation a two-point characteristic.

Hyperfunction of the parathyroid glands is known as hyperparathyroidism. This condition leads to excessive release of parathyroid hormone (PTH), which can be appropriate (secondary or quaternary hyperparathyroidism) or inappropriate (primary, tertiary, or quaternary hyperparathyroidism) relative to the calcium level.

Hypoparathyroidism can occur as a complication of thyroidectomy when the parathyroid glands, which are difficult to locate, are accidentally damaged or removed.

Inflammation of the parathyroid glands is called parathyroiditis.

The functional diagnosis of the parathyroid glands is performed by measuring the parathyroid hormone (PTH) level, which should always be interpreted in relation to the calcium level. For localization diagnostics, ultrasound (sonography) of the parathyroid glands, magnetic resonance imaging (MRI), or a MIBI (methoxyisobutylisonitrile) scan can be used.