medical specialist vs Dr.

medical specialist and Dr. both are nouns.

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

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

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

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