Parathyroid

The parathyroid glands are small, typically four in number, and located behind the thyroid gland. Their primary function is to regulate calcium homeostasis by producing parathyroid hormone (PTH), which increases blood calcium levels by acting on the bones, kidneys, and intestines. Understanding the normal structure of the parathyroid glands and their associated pathologies is critical for diagnosing and managing calcium imbalances.

NORMAL HISTOLOGY OF THE PARATHYROID GLANDS:

The parathyroid glands consist of two main cell types (in connective tissue):

• Chief Cells: These are the most abundant cells in the parathyroid glands and are responsible for producing PTH. Chief cells appear small and dark under the microscope.
• Oxyphil Cells: These are larger and less numerous than chief cells, with a more eosinophilic (pink) cytoplasm. Their function is not well understood, but they increase in number with age.
• Adipose Tissue: As individuals age, adipose tissue progressively infiltrates the parathyroid gland.

The normal architecture of the parathyroid glands is critical for the production and regulation of PTH, and any disruptions to this structure can lead to disorders of calcium metabolism.

PATHOLOGY OF THE PARATHYROIDS:

The parathyroid glands play a crucial role in maintaining calcium homeostasis by secreting parathyroid hormone (PTH). When the regulation of PTH is disrupted, it can lead to hyperparathyroidism, a condition characterized by excessive secretion of PTH. This overproduction of PTH leads to elevated calcium levels in the blood (hypercalcemia) and causes various systemic symptoms, ranging from kidney stones to bone pain and neuropsychiatric disturbances. There are three main types of hyperparathyroidism:

• Primary Hyperparathyroidism: This occurs due to overproduction of PTH from the parathyroid glands themselves, commonly caused by a parathyroid adenoma or, less frequently, parathyroid hyperplasia.
  • Parathyroid Hyperplasia: This condition involves an increase in the number of cells in all four glands (left image below). Histology shows an expansion of both chief and oxyphil cells with minimal adipose tissue (right image below). Treatment often involves surgical removal of three and a half glands, with the remaining portion implanted in a different location (e.g., the arm).

• • Parathyroid Adenoma: A benign tumor affecting one parathyroid gland, resulting in excessive PTH production. Histologically, the adenoma consists of monotonous neoplastic cells, frequently with a rim of normal parathyroid tissue and decreased adipose tissue (image below).

• Secondary Hyperparathyroidism:
  • This condition results from chronic stimulation of the parathyroid glands due to hypocalcemia, typically caused by chronic kidney disease. The failing kidneys are unable to convert vitamin D to its active form or adequately excrete phosphate, leading to hypocalcemia and compensatory overproduction of PTH.

• Tertiary Hyperparathyroidism:
  • This develops after long-standing secondary hyperparathyroidism, when the parathyroid glands become autonomously hyperactive, producing PTH even when calcium levels are normal or elevated.