Anatomy and Histology of the Parathyroid Glands

The parathyroid glands are small endocrine glands located in the neck, behind the thyroid gland. Despite their name, the parathyroid glands are separate from the thyroid gland and have distinct functions. The human body typically has four parathyroid glands, two on each side of the thyroid gland.

The main function of the parathyroid glands is to produce and secrete parathyroid hormone (PTH), a hormone that plays a crucial role in regulating the levels of calcium and phosphorus in the blood. PTH acts on various target organs, including the bones, kidneys, and intestines, to maintain the balance of these minerals.

One of the primary actions of PTH is to increase the concentration of calcium in the blood. It accomplishes this by stimulating the release of calcium from the bones, where it is stored, into the bloodstream. PTH also enhances the reabsorption of calcium in the kidneys, preventing its excretion in urine. Additionally, PTH promotes the activation of vitamin D, which aids in the absorption of calcium from the intestines.

The regulation of PTH secretion is tightly controlled by a negative feedback mechanism. When calcium levels in the blood are low, the parathyroid glands are stimulated to release PTH, which acts to raise calcium levels. Once calcium levels reach the appropriate range, PTH secretion is inhibited, preventing excessive calcium elevation.

Disorders of the parathyroid glands can lead to significant imbalances in calcium and phosphorus levels. One common disorder is hyperparathyroidism, in which the parathyroid glands produce excessive amounts of PTH. This can result in increased calcium levels in the blood, leading to symptoms such as fatigue, kidney stones, and bone pain. On the other hand, hypoparathyroidism, characterized by insufficient PTH production, can cause low levels of calcium in the blood, leading to muscle cramps, tingling sensations, and seizures.

Surgical removal of the parathyroid glands, often necessary in cases of parathyroid tumors or hyperplasia, requires careful preservation of normal gland tissue to maintain proper calcium regulation. This is particularly important to prevent postoperative hypoparathyroidism.

In conclusion, the parathyroid glands are small endocrine glands located behind the thyroid gland. They produce and secrete parathyroid hormone (PTH), which plays a vital role in maintaining calcium and phosphorus balance in the body. Disorders of the parathyroid glands can lead to abnormalities in these mineral levels, necessitating medical intervention. Understanding the function and regulation of the parathyroid glands contributes to our knowledge of endocrine physiology and related health conditions.

References:

  • Bilezikian, J. P., Khan, A. A., Potts Jr, J. T., & Brandi, M. L. (2018). Hypoparathyroidism in the adult: Epidemiology, diagnosis, pathophysiology, target-organ involvement, treatment, and challenges for future research. Journal of Bone and Mineral Research, 33(4), 507-521.
  • Silverberg, S. J., & Bilezikian, J. P. (2018). The diagnosis and management of asymptomatic primary hyperparathyroidism. Nature Reviews Endocrinology, 14(2), 115-125.

Introduction

The parathyroid glands are small, oval-shaped endocrine glands located near the thyroid gland in the neck. Despite their small size, these glands play a crucial role in maintaining calcium and phosphate homeostasis in the body. In this article, we will explore the anatomy and histology of the parathyroid glands.

1. Number and Location

Number

Typically, there are four parathyroid glands in humans. They are usually divided into two pairs: the superior parathyroid glands and the inferior parathyroid glands. However, the number and location of these glands can vary among individuals.

Location

The superior parathyroid glands are usually located on the posterior surface of the thyroid gland, while the inferior parathyroid glands are found below the superior glands, near the lower poles of the thyroid.

2. Gross Anatomy

Size and Shape

The parathyroid glands are very small, with an average size of about 5-6 mm in diameter. They are usually oval or bean-shaped.

Color and Consistency

The parathyroid glands have a yellowish-brown color and a soft, glandular consistency.

3. Microscopic Anatomy (Histology)

Cell Types

The parathyroid glands are composed of two main cell types:- Chief Cells: These cells make up the majority of the parathyroid tissue and are responsible for producing and secreting parathyroid hormone (PTH). PTH plays a crucial role in regulating calcium and phosphate levels in the blood.- Oxyphil Cells: These cells are larger and less numerous than chief cells. Their precise function is not yet fully understood, but they may have a role in calcium metabolism.

Arrangement

The chief cells and oxyphil cells are arranged in clusters or cords, separated by thin connective tissue septa. Blood vessels and capillaries also course through the gland, supplying oxygen and nutrients to the cells.

Stroma

The parathyroid glands are encapsulated by a thin fibrous capsule that surrounds and protects the glandular tissue. Inside the capsule, the gland is supported by a network of reticular fibers and blood vessels.

4. Blood Supply and Innervation

Blood Supply

The parathyroid glands are highly vascularized. They receive their blood supply from branches of the inferior thyroid arteries, which are branches of the external carotid arteries.

Innervation

The parathyroid glands are innervated by branches of the autonomic nervous system, specifically sympathetic and parasympathetic fibers. These fibers help regulate the secretion of parathyroid hormone.

Function of the Parathyroid Glands

Parathyroid Hormone (PTH) Production

The primary function of the parathyroid glands is to produce and secrete parathyroid hormone (PTH). PTH is a key regulator of calcium and phosphate metabolism in the body.

Regulation of Calcium Levels

PTH increases blood calcium levels through several mechanisms:

  1. Bone Resorption: PTH stimulates osteoclasts, which break down bone tissue, releasing calcium into the bloodstream.
  2. Renal Reabsorption: PTH increases calcium reabsorption in the kidneys, reducing the amount of calcium excreted in urine.
  3. Intestinal Absorption: PTH indirectly increases calcium absorption in the intestines by stimulating the production of active vitamin D (calcitriol) in the kidneys.

Phosphate Regulation

PTH also influences phosphate metabolism by reducing phosphate reabsorption in the kidneys, leading to increased phosphate excretion in urine. This helps to balance the levels of calcium and phosphate in the blood.

Regulation of Parathyroid Function

Feedback Mechanism

The secretion of PTH is regulated by a negative feedback mechanism based on blood calcium levels. When calcium levels are low, PTH secretion increases, and when calcium levels are high, PTH secretion decreases. This feedback loop ensures that calcium levels remain within a narrow, optimal range.

Role of Vitamin D

Vitamin D is crucial for the regulation of calcium and phosphate balance. Active vitamin D (calcitriol) enhances the intestinal absorption of calcium and phosphate, and it also modulates PTH secretion. Adequate levels of vitamin D are necessary for the proper functioning of the parathyroid glands.

Clinical Significance

Hyperparathyroidism

Hyperparathyroidism is a condition characterized by excessive production of PTH. It can be classified into primary, secondary, and tertiary hyperparathyroidism:

  1. Primary Hyperparathyroidism: Often caused by a benign tumor (adenoma) in one of the parathyroid glands, leading to excessive PTH secretion and hypercalcemia (high blood calcium levels).
  2. Secondary Hyperparathyroidism: Results from chronic hypocalcemia (low blood calcium levels), often due to chronic kidney disease or vitamin D deficiency, causing compensatory overproduction of PTH.
  3. Tertiary Hyperparathyroidism: Occurs when secondary hyperparathyroidism becomes prolonged and the parathyroid glands become autonomously hyperactive, even after the initial cause is addressed.

Hypoparathyroidism

Hypoparathyroidism is characterized by insufficient production of PTH, leading to hypocalcemia (low blood calcium levels). It can result from surgical removal of the parathyroid glands, autoimmune destruction, or genetic disorders. Symptoms of hypoparathyroidism include muscle cramps, tetany (involuntary muscle contractions), and seizures.

Diagnostic Evaluation

Diagnosis of parathyroid disorders typically involves measuring blood levels of calcium, phosphate, and PTH. Imaging studies, such as ultrasound, sestamibi scan, or CT, may be used to localize abnormal parathyroid glands.

Treatment

Treatment of parathyroid disorders depends on the underlying cause:

  • Surgery: Parathyroidectomy (surgical removal of one or more parathyroid glands) is the primary treatment for primary hyperparathyroidism.
  • Medication: In cases of secondary hyperparathyroidism, treatment may include vitamin D supplements, phosphate binders, and medications that decrease PTH secretion.
  • Calcium and Vitamin D Supplementation: Essential for managing hypoparathyroidism, along with recombinant PTH therapy in some cases.

Conclusion

The parathyroid glands, though small in size, play a crucial role in maintaining calcium and phosphate homeostasis in the body. Their anatomy and histology are specialized to support the production and secretion of parathyroid hormone. Understanding the structure and function of these glands helps in diagnosing and managing disorders related to calcium metabolism.

FAQs Parathyroid Glands

What are the parathyroid glands?

The parathyroid glands are four small endocrine glands located in the neck, usually behind the thyroid gland. They play a crucial role in regulating calcium and phosphate levels in the body.

What is the function of the parathyroid glands?

The main function of the parathyroid glands is to produce parathyroid hormone (PTH). PTH helps maintain proper blood calcium levels by promoting calcium absorption from food, calcium reabsorption in the kidneys, and the release of calcium from bone.

Where are the parathyroid glands located?

The parathyroid glands are typically located behind the thyroid gland, with two glands on either side of the thyroid. They are usually found near the upper and lower poles of the thyroid.

What are the common parathyroid gland disorders?

The most common parathyroid gland disorders include:

  • Hyperparathyroidism – excess production of PTH, leading to high blood calcium levels
  • Hypoparathyroidism – underproduction of PTH, leading to low blood calcium levels
  • Parathyroid adenomas – benign tumors of the parathyroid glands

How are parathyroid gland disorders diagnosed?

Parathyroid gland disorders are typically diagnosed through blood tests to measure calcium and PTH levels, as well as imaging tests like ultrasound, sestamibi scans, or CT scans.

What are the treatment options for parathyroid gland disorders?

Treatment depends on the specific disorder, but may include medications, dietary changes, or surgery to remove all or part of the affected parathyroid gland(s).