Yea theres quite a bit on 3,4-D effects on binding to SHBG. Additionally its not the only lignan in UD to do so but does appear to be the strongest one:
"The natural lignans (-)-3,4-divanillyltetrahydrofuran (1), (-)-matairesinol (2), (-)-secoisolariciresinol (3), (+/-)-enterolactone (4)...