Parathyroid hormone (PTH) is a peptide hormone secreted by the parathyroid glands that regulates calcium and phosphate balance by increasing bone resorption, enhancing renal calcium reabsorption, and stimulating vitamin D activation. Clinically, PTH is crucial in diagnosing and managing disorders of calcium metabolism, such as primary hyperparathyroidism, which can cause hypercalcaemia and bone disease.