How does Mirena (levonorgestrel) IUD affect hormone levels?

How Mirena IUD Affects Hormone Levels

The Mirena IUD releases low-dose levonorgestrel locally, resulting in minimal systemic hormonal effects—most women (70-93%) continue to ovulate normally, though with slightly reduced progesterone and estradiol levels, while the primary contraceptive mechanism is local thickening of cervical mucus and endometrial changes rather than ovulation suppression. 1

Primary Mechanism and Systemic Absorption

The levonorgestrel IUD works primarily through local effects rather than systemic hormonal suppression:

• The device prevents fertilization by inhibiting sperm motility and thickening cervical mucus, with all mechanisms occurring before implantation 1
• Systemic levonorgestrel levels remain relatively low, averaging 260 pg/ml in the first three months and declining to 129 pg/ml after one year of use 2
• After 6 years of use, serum levonorgestrel levels are maintained at mean levels of 314 pmol/L in ovulatory cycles 3

Impact on Ovulation

Most women maintain ovulatory function despite using Mirena:

• After long-term use (6-7 years), 70-93% of cycles remain ovulatory based on progesterone levels 4, 3
• However, only 58% of these "ovulatory" cycles show completely normal follicular growth and rupture on ultrasound 4
• Follicular cysts and luteinization of regressing follicles occur in approximately 42% of ovulatory cycles, though these typically resolve spontaneously within 45 days 4

Specific Hormonal Changes

The systemic hormonal effects are subtle but measurable:

Progesterone

• Luteal phase progesterone levels are lower in Mirena users compared to copper IUD users, though this difference may not reach statistical significance 5
• Peak progesterone levels during ovulatory cycles are reduced compared to controls 6, 5

Estradiol and LH

• Preovulatory estradiol levels are lower in levonorgestrel IUD users than in copper IUD users 4, 5
• LH peak levels are also reduced, though differences may not be statistically significant 4, 5

Sex Hormone Binding Globulin (SHBG)

• SHBG capacity is decreased in Mirena users compared to copper IUD users 4, 5
• This reflects some degree of systemic progestin effect, though minimal 5

Clinical Implications

The minimal systemic hormonal effects make Mirena particularly useful in specific populations:

• It provides an excellent option for patients who have safety concerns about estrogen use 7
• It is particularly useful for adolescents with medical conditions requiring long-term menstrual suppression where estrogen is contraindicated 1
• Many women experience lighter periods or amenorrhea (occurring in approximately 20% after one year), which is due to local endometrial effects rather than ovulation suppression 1, 2

Important Caveats

Cervical mucus effects are not the sole mechanism:

• Good cervical mucus was observed in 69% of ovulatory cycles in long-term users, indicating that cervical mucus changes cannot be the main mechanism of action 4
• The contraceptive effect after long-term use is mainly due to local endometrial changes, with much less effect on ovarian function 3

Cycle irregularities are common initially:

• Intermenstrual spotting occurs frequently during the first 60 days of treatment 6, 2
• Cycles may be longer than baseline, particularly in the first year 6