The analysis of Tetrahydrobiopterin (BH4) in urine by High-Performance Liquid Chromatography (HPLC) is an important laboratory test used to determine the levels of this bioactive compound in urine. Tetrahydrobiopterin is considered an essential substance in the process of producing neurotransmitters such as serotonin and dopamine. The results of this analysis help in diagnosing several metabolic disorders, such as BH4 deficiency, which may lead to issues in amino acid metabolism. The procedure involves collecting a urine sample, then analyzing it using chromatography techniques to obtain accurate results regarding BH4 concentration. The accuracy and reliability of the results are critical for guiding appropriate treatment and assessing the patient's health condition.