Estriol-d2 is primarily used as an internal standard in analytical chemistry for the quantification of estriol in various biological samples. Its deuterated form allows for precise and accurate measurement in mass spectrometry, ensuring minimal interference from the sample matrix. This compound is particularly valuable in clinical research and diagnostics, where it aids in monitoring estriol levels during pregnancy, assessing fetal health, and studying hormonal imbalances. Additionally, it plays a role in pharmaceutical development for validating methods related to estrogen-based therapies. Its stability and isotopic labeling make it a reliable tool for trace analysis in complex biological systems.