follow up vs follow-up

follow up is not a noun while follow-up is a noun.

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

follow up (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

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