TPH1 and irritable bowel syndrome: RCT - Randomized controlled trial, TCA - Tricyclic antidepressants, IBS-D - Irritable bowel syndrome diarrhea-predominant, 5HT - serotonin, TPH - Tryptophan hydroxylase, SERT- Serotonin reuptake transporter, TP - Traditional pharmacotherapy, HRQOL - Health-related quality of life