The parathyroid glands are small endocrine glands located in the neck, usually positioned behind the thyroid gland. These glands play a critical role in regulating calcium homeostasis in the blood through the secretion of parathyroid hormone (PTH).
Structure of the Parathyroid Gland
Location and Structure
Position: The parathyroid glands are usually found on the posterior aspect of the thyroid gland.
There are typically four glands, two superior and two inferior, though the number can vary from two to six.
“Diagram of the human neck structures, showing the four typical locations of the parathyroid glands (green) behind the lateral lobes of the thyroid gland (orange).”
Shape and Size:
Each gland is oval or bean-shaped.
They are small, measuring about 3-8 mm in length, 2-5 mm in width, and 1-2 mm in thickness.
Each gland weighs approximately 30-50 mg.
2. Histology:
Chief Cells: These are the primary cells responsible for producing and secreting PTH. They are small, polygonal cells with prominent nuclei.
Oxyphil Cells: These cells are larger and less numerous than chief cells. Their function is not completely understood, but they may be involved in the secretion of PTH in certain conditions.
Adipose Tissue: Fat cells are interspersed throughout the parathyroid glands, increasing in number with age.
Functions of the Parathyroid Gland
1. Calcium Homeostasis:
Parathyroid Hormone (PTH): The primary function of the parathyroid glands is to maintain an optimal level of calcium in the blood. PTH plays a crucial role in this process.
2. Mechanisms of PTH Action:
Bone Resorption: PTH stimulates osteoclasts, which break down bone tissue, releasing calcium and phosphate into the bloodstream.
Kidney Function: PTH increases calcium reabsorption in the renal tubules, reducing calcium excretion in urine. It also stimulates the conversion of 25-hydroxyvitamin D to its active form, calcitriol, which in turn enhances intestinal absorption of calcium.
Intestinal Absorption: By promoting the production of active vitamin D (calcitriol), PTH indirectly increases the absorption of calcium from the intestines.
3. Phosphate Homeostasis:
PTH decreases phosphate reabsorption in the kidneys, leading to increased excretion of phosphate in urine. This action helps to prevent the formation of calcium-phosphate crystals in the body.
Disorders of the Parathyroid Gland
1. Hyperparathyroidism:
Primary Hyperparathyroidism: Excessive secretion of PTH due to abnormal parathyroid glands, often caused by a benign tumor (adenoma), hyperplasia, or rarely, carcinoma.
Symptoms: Hypercalcemia, bone pain, osteoporosis, kidney stones, excessive urination, abdominal pain, fatigue, depression.
Diagnosis: Elevated serum calcium and PTH levels, imaging studies (ultrasound, Sestamibi scan).
Treatment: Surgical removal of the overactive gland(s) (parathyroidectomy).
Secondary Hyperparathyroidism: Excessive secretion of PTH in response to chronic low levels of calcium, often due to chronic kidney disease or vitamin D deficiency.
Symptoms: Similar to primary hyperparathyroidism but typically associated with underlying kidney disease.
Diagnosis: Elevated PTH with low or normal serum calcium, high phosphate levels in chronic kidney disease.
Treatment: Addressing the underlying cause, vitamin D supplementation, phosphate binders, calcimimetics.
Tertiary Hyperparathyroidism: Persistent hypersecretion of PTH after the correction of long-standing secondary hyperparathyroidism, often in patients with chronic kidney disease.
Symptoms: Similar to primary hyperparathyroidism.
Diagnosis: Elevated PTH and calcium levels after treatment of the underlying cause.
Treatment: Parathyroidectomy.
2. Hypoparathyroidism:
Insufficient secretion of PTH leading to low levels of calcium in the blood.
Causes: Surgical removal of or damage to the parathyroid glands, autoimmune destruction, genetic disorders (e.g., DiGeorge syndrome), magnesium deficiency.
Symptoms: Hypocalcemia, muscle cramps, tetany (involuntary muscle contractions), paresthesias (tingling in fingers and toes), seizures, cardiac arrhythmias.
Diagnosis: Low serum calcium, low PTH levels, high phosphate levels.
Treatment: Calcium and vitamin D supplementation, recombinant PTH (in severe cases).
3. Pseudohypoparathyroidism:
A genetic disorder where the body is resistant to PTH, leading to symptoms of hypoparathyroidism despite normal or elevated levels of PTH.
Symptoms: Similar to hypoparathyroidism (hypocalcemia, tetany), plus short stature, obesity, and developmental abnormalities in some cases.
Diagnosis: Hypocalcemia, elevated PTH levels, characteristic physical features.
Treatment: Calcium and vitamin D supplementation.
Conclusion
The parathyroid glands are essential for maintaining calcium and phosphate balance in the body. Through the secretion of parathyroid hormone, they regulate bone metabolism, kidney function, and intestinal calcium absorption. Disorders of the parathyroid glands, such as hyperparathyroidism and hypoparathyroidism, can lead to significant metabolic disturbances and require appropriate diagnostic and therapeutic interventions. Understanding the structure, function, and pathologies associated with the parathyroid glands is crucial for the effective management of related endocrine disorders.
