Could refusing medical treatment be considered suicide?

Maybe: Everyday, many people with terminal illness refuse treatment to be allowed to die "naturally" - this is part of compassionate and palliative care. However if you refuse urgent life saving treatment because you really wish to die then this maybe a form of suicide.

It is not: The major world faiths that i know make a clear distinction between willfully ending your own life by your direct action (which many consider to be wrong in most circumstances) and saying "no" to care that might prolong or even save life. In fact, in most faith-based and secular morals, you have a right to refuse -- wise or foolish.