why are auxins present in human urine??

Human urine shows common presence of indole-3-acetic acid (IAA), compound having similar properties to plant hormone, auxin. This fact was discovered first in 1931 by Kogl and Haagen-Smit, five years earlier the discovery of auxin in plants.

 Indole-3-acetic acid (IAA) has a metabolic reason for its generation in human body. It is formed by breakdown of serotonin, mainly in liver and then is excreted in the urine. 

Serotonin is a neurotransmitter synthesised from tryptophan, similar to how auxin is made in plants.The breakdown rate of serotonin into 5-hydroxyindoleacetic acid, increases under some abnormal conditions such as when a person is having certain tumors or cancers.