Used as a stable isotope-labeled internal standard in mass spectrometry-based assays for accurate quantification of estradiol in biological samples such as serum, plasma, and urine. Its primary application is in clinical diagnostics and research settings to measure endogenous estradiol levels with high precision by compensating for matrix effects and instrument variability. Also employed in metabolic studies to trace the fate of estradiol in pharmacokinetic and metabolic pathways without altering the biological behavior of the molecule.