Mirena, a small, flexible T-shaped IUD, releases a hormone called Levonorgestrel, a hormone that effectively prevents pregnancy. Mirena is placed in a woman's uterus and is inserted by a doctor during an outpatient office visit. The problem with Mirena is... that there are a lot of problems that were identified when clinical trials for Mirena began in 1985 (when Mirena was called Levonova).