Parathyroid Hormone (PTH) is an 84-amino acid peptide hormone produced by the parathyroid glands that plays a crucial role in maintaining calcium homeostasis. This test measures intact PTH levels in the blood to evaluate parathyroid function and diagnose disorders of calcium metabolism. PTH acts on bones, kidneys, and intestines to increase blood calcium levels by stimulating bone resorption, enhancing renal calcium reabsorption, and promoting vitamin D activation. Elevated PTH levels can indicate primary hyperparathyroidism (parathyroid gland overactivity), secondary hyperparathyroidism (compensatory response to low calcium), or parathyroid tumors. Low PTH levels suggest hypoparathyroidism or surgical removal of parathyroid tissue. This test is essential for evaluating patients with abnormal calcium levels, monitoring calcium metabolism in kidney disease, and guiding treatment decisions for parathyroid disorders.