Losartan (D4 Carboxylic Acid) is primarily used as a labeled internal standard in analytical chemistry for the quantification of losartan and its metabolites in biological samples. It plays a critical role in pharmacokinetic studies, enabling researchers to accurately measure drug concentrations in plasma, urine, or other tissues. This is essential for understanding drug absorption, distribution, metabolism, and excretion. Additionally, it is employed in quality control processes during the manufacturing of losartan-based pharmaceuticals to ensure consistency and compliance with regulatory standards. Its deuterated form enhances precision in mass spectrometry analyses, reducing interference and improving detection sensitivity.