Parathyroid hormone (PTH) is produced and released by the parathyroid glands, four small glands located in the neck behind the thyroid gland. PTH maintains the balance of certain minerals in the bloodstream, important for processes like muscle contraction, nerve function, and bone structure.
What is Parathyroid Hormone (PTH)?
Parathyroid hormone is a polypeptide hormone, a chain of 84 amino acids. This specific sequence gives PTH its unique structure, allowing it to interact with specific receptors in the body.
PTH is produced by chief cells within the parathyroid glands. Its release is triggered by a decrease in calcium levels in the blood. The parathyroid glands contain calcium-sensing receptors that detect these changes. Once released, PTH circulates in the blood to restore calcium balance. The hormone has a short half-life, estimated at 2 to 5 minutes.
How PTH Regulates Calcium and Phosphate
The balance of calcium and phosphate in the blood is maintained through a process called homeostasis. PTH is a primary regulator, increasing calcium levels in the blood and generally decreasing phosphate levels. This regulation is a continuous process that responds to the body’s needs.
When blood calcium levels drop, the parathyroid glands release more PTH, which acts on various organs to raise calcium. Conversely, when blood calcium levels are high, PTH secretion is suppressed, maintaining calcium within a narrow range, typically 9.0 to 10.1 mg/dL.
PTH also influences phosphate. While it raises calcium, it promotes the excretion of phosphate by the kidneys. This is important because high phosphate can bind with calcium, reducing free calcium in the blood.
PTH’s Effects on Body Systems
PTH accomplishes its role in mineral regulation by acting on several body systems. Its actions on bones, kidneys, and intestines adjust calcium and phosphate levels.
In the bones, PTH stimulates the release of calcium and phosphate into the bloodstream. It does this by indirectly promoting osteoclast activity, which are cells responsible for breaking down bone. PTH binds to osteoblasts, the cells that build bone, which then signal the osteoclasts to resorb bone, releasing stored minerals.
In the kidneys, PTH has multiple effects. It increases the reabsorption of calcium into the blood, preventing its loss in urine. PTH reduces the reabsorption of phosphate, leading to increased phosphate excretion. PTH also stimulates the production of the active form of vitamin D, called calcitriol.
The activated form of vitamin D, produced with PTH’s help, influences the intestines. Calcitriol enhances the absorption of calcium and phosphate from food. This indirect action of PTH through vitamin D contributes to overall calcium and phosphate balance.
Conditions Related to PTH Imbalance
When parathyroid hormone levels are not properly regulated, it can lead to health conditions affecting mineral balance. These conditions often manifest with symptoms related to abnormal calcium levels.
Hyperparathyroidism occurs when there is an overproduction of PTH, often due to a benign tumor in one of the parathyroid glands. This leads to high calcium levels in the blood, a condition known as hypercalcemia. Symptoms can include kidney stones, bone pain, increased thirst, fatigue, and issues with mental function such as depression or memory loss.
Conversely, hypoparathyroidism results from insufficient PTH production, leading to abnormally low calcium levels in the blood. This condition can arise from parathyroid gland damage, often following neck surgery, or from autoimmune disorders. Symptoms associated with low calcium include muscle cramps, tingling sensations, numbness, and in severe cases, seizures.