followup vs follow up

followup is a noun but follow up is not a noun.

Word	Noun	Adjective	Verb	Adverb
followup	Yes	No	No	No
follow up	No	No	Yes	No

followup (noun)	follow up (noun)
a subsequent examination of a patient for the purpose of monitoring earlier treatment
an activity that continues something that has already begun or that repeats something that has already been done
a piece of work that exploits or builds on earlier work

followup (verb)	follow up (verb)
	pursue to a conclusion or bring to a successful issue
	increase the effectiveness or success of by further action