Levonorgestrel is the Progestin in Mirena IUD
The Mirena IUD contains levonorgestrel, which is a synthetic progestin (not progesterone itself), and it is the only hormone used in all currently available hormonal IUDs. 1
Key Facts About the Hormone
Levonorgestrel is a progestin-only contraceptive, meaning it contains no estrogen component whatsoever 1
The Mirena device releases 20 micrograms of levonorgestrel per 24 hours into the uterine cavity 2, 3
The total levonorgestrel content in the Mirena device is 52 mg, which provides continuous hormone release for at least 5 years (FDA-approved duration), though efficacy extends to 7-8 years 1
Clinical Significance of Progestin-Only Formulation
The absence of estrogen makes levonorgestrel IUDs suitable for women with contraindications to estrogen, including those with:
- Antiphospholipid antibodies
- History of thromboembolism
- Decompensated cirrhosis
- Cardiovascular risk factors 1
The progestin-only nature eliminates estrogen-associated risks while maintaining highly effective contraception with a failure rate of less than 1% 1
Mechanism of Action
Levonorgestrel works primarily by preventing fertilization through inhibiting sperm motility and thickening cervical mucus, with all mechanisms occurring before implantation 1
The local intrauterine delivery of levonorgestrel provides endometrial suppression, which explains its therapeutic benefits for heavy menstrual bleeding 3
Important Distinction
- Levonorgestrel is a synthetic progestin, not natural progesterone—this distinction matters for counseling patients who may confuse the two terms 1