newsletter vs write up

newsletter and write up both are nouns.

Word	Noun	Adjective	Verb	Adverb
newsletter	Yes	No	No	No
write up	Yes	No	Yes	No

As nouns, write up is a hypernym of newsletter; that is, write up is a word with a broader meaning than newsletter:

newsletter: report or open letter giving informal or confidential news of interest to a special group
write up: a short account of the news

Other hypernyms of newsletter include account, news report, report, story.

newsletter (noun)	write up (noun)
report or open letter giving informal or confidential news of interest to a special group	a short account of the news

newsletter (verb)	write up (verb)
	bring to public notice by writing, with praise or condemnation
	put into writing; write in complete form