The hormone in mirena does not stay 'down there'. That is a completely illogical and stupid thing for a doctor to say. The uterous, like every other organ in the body has a blood supply. The uterous is not a 'sealed unit' from which nothing can escape. The hormone in mirena enters the blood via the blood vessels which supply the uterous, and the hormone travels around the whole body via the blood system.
I explained this to my ex-doc when he tried to tell me the same guff of mirena not being a systemic hormone. I laughed at him and asked him how on earth he got his medical degree if he didn't understand basic anatomy.