Is merely getting up for Suhur before Fajr with the intention of making up a fast (qada') sufficient as a nia (intention), even without uttering it or specifying it for qada'?

Intention (niyyah (definition)) is an act of the heart that does not require verbalization. So, if you intended to fast a make-up day (qada') before sleeping, this is sufficient, and you are not required to renew the intention or verbalize it at the time of suhoor (definition). Making the intention overnight is a condition for obligatory fasting, and this occurs during the night, between sunset and dawn, based on the Prophet's (peace be upon him) saying: "Whoever does not firmly intend to fast before dawn, there is no fast for him."