Intestinal glucuronidation metabolism may have a greater impact on oral bioavailability than hepatic glucuronidation metabolism in humans:
A study with raloxifene, substrate for UGT1A1, 1A8, 1A9, and 1A10

The kinetic impact of intestinal glucuronidation metabolism on oral bioavailability was assessed using reported human data of raloxifene, of which oral bioavailability was only 2%. Presystemic intestinal availability was 5.4%, whereas fraction absorbed and hepatic availability were 63% and 59.3%, respectively. Thereby, UGTs in the intestine may play important roles in the first-pass metabolism.

International Journal of Pharmaceutics