There is no need to test at all with
in place. You will get, as many women do, a rise in BBT and then a drop when your period is here.
My advice is don't make yourself more nervous and use BBT when an IUD is in place. It is meaningless.
Depending on the type of IUD you have in place, you may get little to no periods, anyway. If you are concerned, you can take a pregnancy test once a month, just to assure your own peace of mind, but that really isn't necessary, either.