Hydroxychloroquine-d4 sulfate (HCQ-d4) is primarily used as an internal standard in analytical chemistry for the quantification of hydroxychloroquine in various biological samples. Its deuterated form ensures minimal interference with the analyte, providing accurate and reliable measurements in mass spectrometry and other detection methods. This compound is particularly valuable in pharmacokinetic studies, helping researchers understand the absorption, distribution, metabolism, and excretion of hydroxychloroquine in the body. Additionally, it plays a critical role in clinical research, supporting the development and validation of assays for therapeutic drug monitoring and ensuring precise dosing in patients undergoing hydroxychloroquine treatment.