Mirena IUD Composition
The Mirena IUD consists of a T-shaped polyethylene frame with a cylindrical reservoir on the vertical stem containing 52 mg of levonorgestrel mixed with polydimethylsiloxane, covered by a rate-limiting silastic membrane that releases 20 micrograms of levonorgestrel per 24 hours into the uterine cavity. 1, 2
Physical Structure
- The device is built on a plain Nova-T frame design with a silastic (polydimethylsiloxane) reservoir attached to the vertical arm 3
- The reservoir is impregnated with levonorgestrel and covered with a rate-limiting silastic membrane to control hormone release 3
- The plastic frame measures 32 mm in length and is T-shaped to conform to the uterine cavity 2
Hormonal Component
- The device contains 52 mg of levonorgestrel total, which is a progestin-only hormone with no estrogen component 1, 4
- The release rate is approximately 20 micrograms of levonorgestrel per 24 hours, maintained for at least 5 years (and effective up to 7 years) 1, 3
- This local delivery system maintains very low circulating levels of levonorgestrel while achieving high concentrations directly in the uterine cavity 5
Clinical Significance of Composition
- The progestin-only formulation avoids all estrogen-associated risks, making it suitable for women with contraindications to estrogen including cardiovascular risk factors, history of thromboembolism, or migraine with aura 1, 4
- The silastic membrane technology allows for steady, controlled hormone release that provides both contraceptive efficacy (failure rate <1%) and therapeutic benefits for heavy menstrual bleeding 1, 3