He had a close relationship with the domain of Satsuma, and Ikuhime, the lawful wife of Tadahiro was the daughter of Narioki SHIMAZU, the lord of Satsuma Domain (in fact she was the daughter of Narinobu SHIMAZU, the former lord of the domain).
薩摩藩と関係が深く、忠熙の正室郁姫は薩摩藩主島津斉興の娘（実は前藩主・島津斉宣の娘）である。