Tryptophan metabolism plays an important role in the metabolic control of immunological and neuropsychological processes. Determination of tryptophan and its metabolites are required to understand the pathophysiological mechanisms of various diseases associated with tryptophan metabolism and to evaluate the therapeutic approaches. The relationship between kynurenine (KYN) pathway metabolites and disease: The degradation of tryptophan associated with inflammation suggests that assessment of the metabolomic profile of patients with COVID-19 may assist in disease severity classification and even guide clinical decisions. Plasma levels of tryptophan and some of its metabolites via the kynurenine pathway vary according to the severity of COVID-19. It was determined that while tryptophan concentrations decreased significantly depending on the severity of the disease, kynurenine pathway metabolites such as 3-hydroxykynurenine increased. The gut microbiota has also been shown to modulate the kynurenine pathway, thereby influencing the physiology of the digestive system and central nervous system. Indeed, the KYN pathway is thought to play a key role in the modulation of neurotransmission and immune functions. Fluctuations in the kynurenine pathway are associated with many psychiatric and gastrointestinal disorders. Metabolic alterations in the kynurenine pathway may cause various biological responses in depression. Increasing evidence has been gathered regarding the relationship between the activation of the KYN pathway and the onset of depression.