After Interview or Shadow? When to send thank you letter...

Both. After both occasions, you have reason to thank someone for their time and effort.

Send it now. Thank them for the interview and the opportunity to shadow and tell them that you are looking forward to shadowing. Don't wait until you shadow. Also, send a thank you note after the shadowing experience too.

I suggest e-mailing immediately. Handwritten notes are nice, but take too long to arrive and sometimes get held up in the hospital mail system.

I agree with the previous poster; send a thank you after every face to face visit.

I set a thank you note, hand written, snail mail, right when I got home from my interview (I already had the shadow lined up for a few days later). They thanked me at my shadow for my thank you note. :-) It was nice!

I got the job.

I prefer handwritten over email (even though email is fine). But, since you have a week before your shadow experience, you have the few days to get the letter there. I sent one as soon as my shadowing experience was over (I did a shadow experience a month before my interview) I had it already addressed and just wrote in the card in the parking lot after the shadowing experience and mailed it at the post office in the same town.

When I went to interview a month later, I had gotten an offer on the spot but I still went to get 6 cards (I interviewed with 6 people!) and came back an hour after the interview to drop off the thank you cards.

You have a little time to snail mail but I would do it asap. If not, an email is fine but definitely send SOMETHING before the shadow experience. That way you stay on their mind before you come back.

I sent an email and a thank you note to everyone who interviewed me. I got the job about two days after sending the thank you notes and my manager did thank me for the note the next time I spoke to her. It definitely keeps you in mind. It also shows that you are courteous, aware of current professional behavior, and if you send it promptly it shows that you are on top of things. Definitely send a thank you note for everything.