How does the Mirena (levonorgestrel) IUD affect uterine fibroids?

How the Mirena IUD Affects Uterine Fibroids

The Mirena (levonorgestrel) IUD effectively reduces menstrual bleeding associated with uterine fibroids but does not shrink the fibroids themselves, making it an appropriate first-line treatment for bleeding symptoms only. 1, 2

Effect on Bleeding Symptoms

The levonorgestrel IUD demonstrates significant therapeutic benefit for fibroid-related bleeding:

• Most women (60-100%) experience substantial reduction in menstrual blood loss after Mirena insertion, with improvements in hemoglobin, hematocrit, and ferritin levels. 1, 3
• Studies show menstrual blood loss reduction ranging from 77.5% (measured by alkaline hematin test) to 34.5% (measured by pictorial assessment chart) compared to combined oral contraceptives. 4
• Some women with intramural fibroids develop amenorrhea within 12 weeks of insertion, while others experience scanty menstrual bleeding. 5
• Anemic women typically normalize their hemoglobin levels within 6 months of IUD insertion. 5

Effect on Fibroid Size

The Mirena IUD does not reduce fibroid volume. 2, 5

• Ultrasound evaluations consistently show no change in fibroid size after levonorgestrel IUD placement. 5
• The mechanism of action is local endometrial suppression (reducing endometrial thickness to ≤4mm), not fibroid shrinkage. 5
• This contrasts with GnRH agonists, which achieve 18-50% fibroid volume reduction. 1, 2

Risk of Expulsion

Women with uterine fibroids face higher expulsion rates (11%) compared to women without fibroids (0-3%). 1, 3

• Prospective studies report expulsion rates ranging from 0-20% in women with fibroids. 3
• Fibroids can physically displace the IUD within the uterine cavity, contributing to expulsion risk. 6
• Despite this risk, most women who retain the device experience sustained benefit. 3

Clinical Algorithm for Mirena Use in Fibroids

Use the levonorgestrel IUD as first-line therapy when:

• The primary symptom is heavy menstrual bleeding (not bulk symptoms like pelvic pressure or pain). 1, 7
• Fibroid size and location do not significantly distort the uterine cavity. 1
• The patient desires long-term, user-independent hormonal management. 1

Do NOT use the levonorgestrel IUD when:

• Bulk symptoms (pelvic pressure, urinary frequency, constipation) are the primary complaint—these require fibroid volume reduction. 2, 7
• Anatomical distortion of the uterine cavity makes proper IUD placement impossible. 1
• Preoperative fibroid shrinkage is needed before myomectomy—use GnRH agonists instead. 2

Treatment Failure Considerations

Among women requiring hysterectomy after failed Mirena treatment, 86% had significant uterine pathology:

• 68% had leiomyomas, with 10 cases showing IUD displacement by fibroids. 6
• 32% had adenomyosis (often coexisting with fibroids). 6
• Only 13.6% had no identifiable uterine abnormalities. 6

This suggests that large or cavity-distorting fibroids predict Mirena failure, and alternative treatments (GnRH modulators, UAE, or surgery) should be considered earlier in these cases. 6

Positioning Relative to Other Treatments

The levonorgestrel IUD serves as first-line medical management alongside combined oral contraceptives and tranexamic acid for bleeding control. 1, 8, 7

• For bleeding symptoms alone: Mirena is preferred over oral contraceptives due to local mechanism, lower systemic hormone exposure, and superior user independence. 1
• For fibroid shrinkage: GnRH agonists/antagonists are required, achieving 18-50% volume reduction over 3-4 months. 1, 2
• For definitive treatment: Surgical options (myomectomy, hysterectomy) or UAE provide permanent resolution when medical management fails. 1, 7

Common Pitfalls to Avoid

• Do not expect fibroid shrinkage—counsel patients that bleeding will improve but fibroids will not decrease in size. 2, 5
• Assess cavity anatomy before insertion—significant cavity distortion increases expulsion risk and reduces effectiveness. 1, 6
• Monitor for irregular bleeding—while most women improve, some experience persistent irregular bleeding requiring device removal. 5, 3
• Recognize treatment failure early—if bleeding persists beyond 12 weeks or bulk symptoms predominate, transition to second-line therapies (GnRH modulators) or procedural interventions. 7, 6