Mirena IUS Side Effects & Heavy Bleeding: A Patient's Experience

Question
I am 18 years old and had the Mirena IUS inserted 6 months ago. Before that I was on birth control pills. I switched because of blood clotting concerns and spotting. I had heavy periods before the pill and very light while on the  pill. I was told my period should eventually disappear while on the IUS and mine has gotten worse. My period will usually last about 10-12 days with very heavy flow. This happened until about the 5th month. After that i experienced constant spotting. Now into the sixth month i have been experiencing constant bleeding everyday. Sometimes off and on through out the day. Dark brown with clots and this has lasted about 2 weeks after last period. I am sexually active and have not been using protection. I do experience some pain with intercourse but very mild and I get some cramping and pains through the day. I have also been very tired and have been tested for TSH levels which were on the high end of normal. there is a history of thyroid disease in my family.
My question is what would cause such heaving bleeding while on mirena and if it normal to experience this?

Answer
It's not normal to experience the heavy bleeding while on the Mirena, and it's hard to know what would cause it.  The fact that you had light periods while on the pill means that your uterus seems normal and responds normally to hormones.  As you mention, many women will stop having periods after being on the Mirena for a number of months.  If your bleeding seems to gradually be getting better, then I'd probably keep the Mirena and hope the bleeding will eventually stop.  If it's not getting better, then I'd have it removed.  An STD or other infection is another cause for unexplained bleeding, so I'd get checked for that if you haven't.