Mirena and tampons.

I also wanted to mention that I believe Mirena IUD was primarily for women who already have had children. Have you talked with your health care provider about this? Here is some information from the Mirena website:
"Mirena (levonorgestrel-releasing intrauterine system) is recommended for women who have had at least one child. This is because most of the medical research conducted on Mirena for FDA approval was among women who had at least one child." source: http://www.mirena-us.com/mirena_right/index.jsp

Great question!

The IUD is a type of birth control that is inserted into your uterus (by a health care provider), and there are strings that hang out of the opening of your cervix into your vagina (the strings are just barely in your vagina; they are not long enough to be seen).

When you use a tampon, it is plausible that the cotton fibers of the tampon can "catch" the strings of the IUD. I am not sure how often this would happen, and many women with IUDs do successfully use tampons.

The "careful" removal of the tampon would just be necessary. Worst case scenario: you would feel a strong tug, possible pain, if the tampon "caught" the IUD strings, so you just would want to pull the tampon gently out. It would be almost impossible to pull out the IUD from your cervix; it would be extremely painful and the most that could happen is possibly causing the IUD to shift position which could cause pain and would need to be repositioned by your health care provider.

A bathing suit or thong underwear would not create a risk, as they are not inserted inside your vagina, and are instead on the outside of your vulva (labia and vaginal opening).

Does this help? Why are you interested in the IUD as birth control (versus other birth control options)?