Hormone in the Mirena IUD
The Mirena IUD contains levonorgestrel, a synthetic progestin hormone, which is released at a rate of approximately 20 micrograms per 24 hours. 1, 2
Details about Mirena's Hormone Content
- Mirena is a hormonal intrauterine device (IUD) that contains 52 mg of levonorgestrel 1
- The hormone is contained in a silastic reservoir attached to the vertical arm of the device, covered by a rate-limiting silastic membrane that controls the release rate 3
- Initially, the release rate is higher, but stabilizes to approximately 20 micrograms of levonorgestrel per day for at least 5 years 3, 2
- Plasma concentrations of levonorgestrel are highest in the first three months (mean 260 pg/ml) and decrease to lower levels (mean 129 pg/ml) after one year of treatment 4
Mechanism and Local Effects
- The levonorgestrel is released directly into the uterine cavity, causing pronounced endometrial suppression 3
- The dose is low enough that systemic effects are minimal, with negligible impact on ovarian function in most women 3
- This localized hormone delivery system allows for effective contraception with lower systemic hormone exposure compared to oral contraceptives
Duration of Use and Effectiveness
- Mirena is FDA-approved for use for 5-7 years 1
- It has an extremely high contraceptive efficacy with a Pearl index (pregnancy rate) of only 0.09 5, 3
- The low failure rate makes it one of the most effective reversible contraceptive methods available
Other Levonorgestrel IUDs
For comparison, other levonorgestrel-releasing IUDs include:
- Skyla (13.5 mg levonorgestrel) - approved for 3 years
- Liletta (52 mg levonorgestrel) - approved for 3 years
- Kyleena (19.5 mg levonorgestrel) - approved for 5 years 1
Clinical Considerations
- The local release of levonorgestrel provides additional non-contraceptive benefits, including reduction in menstrual bleeding and improvement in dysmenorrhea 2
- Common side effects include changes in bleeding patterns, particularly in the first 3-6 months, which may include irregular spotting or light bleeding 1
- Some women (approximately 28.6%) may develop amenorrhea with prolonged use 6
- The levonorgestrel IUD can be particularly beneficial for women with heavy menstrual bleeding, as it significantly reduces menstrual blood loss 6, 2
Understanding that Mirena contains levonorgestrel is important for patients considering this contraceptive option, especially those with contraindications to certain hormones or those seeking the non-contraceptive benefits of this IUD.