L-Thyroxine-13C9,15N is primarily used in research and diagnostic applications as a stable isotope-labeled compound. It serves as an internal standard in mass spectrometry for the accurate quantification of thyroxine in biological samples. This labeled compound helps in studying thyroid function and metabolism, particularly in cases of thyroid disorders such as hypothyroidism and hyperthyroidism. Additionally, it aids in the development and validation of analytical methods for hormone analysis, ensuring precision and reliability in clinical and pharmaceutical research.