Whom should I tell that I am taking blood thinners?

All Drs, NPs, PAs. You should tell all your healthcare providers and be sure they are aware whenever you are prescribed a new medication or change in dose. The most common blood thinner, warfarin or coumadin, is a tricky medicine that interacts with many medicines. This could affect the potency of the other medicine and/or the thinness (measured by the inr) of the blood.
Everyone medical. 'blood thinners' (or anti-coagulants as the technical term) are extremely useful agents; but have very serious side effects. Some require periodic monitoring through blood testing; others do not. It is important to share this; as well as other medications you take with your health care providers: the more they know about you the better care they provide to you!