specialist vs Dr.

specialist and Dr. both are nouns.

Word	Noun	Adjective	Verb	Adverb
specialist	Yes	No	No	No
Dr.	Yes	No	No	No

As nouns, Dr. is a hypernym of specialist; that is, Dr. is a word with a broader meaning than specialist:

Other hypernyms of specialist include doc, doctor, MD, medico, physician.

specialist (noun)	Dr. (noun)
an expert who is devoted to one occupation or branch of learning	a licensed medical practitioner
practices one branch of medicine	a person who holds Ph.D. degree (or the equivalent) from an academic institution