Used primarily as a labeled internal standard in mass spectrometry and liquid chromatography-tandem mass spectrometry (LC-MS/MS) for the accurate quantification of ethinyl estradiol in biological samples such as plasma, serum, and urine. Its deuterated structure allows for precise differentiation from the non-labeled compound during analytical testing, improving measurement accuracy in pharmacokinetic and metabolic studies. Commonly applied in clinical research, drug development, and bioequivalence trials involving hormonal contraceptives or hormone replacement therapies. Also utilized in environmental testing to trace and quantify estrogenic compounds in water and wastewater due to its stability and detectability.