Irregular uterine bleeding associated with Mirena IUD: The patient has been using a Mirena IUD for 2 years, and it's known that hormonal IUDs can cause changes in menstrual bleeding patterns, including irregular bleeding or spotting, especially in the first few months after insertion but can persist.

Breakthrough bleeding: Common in users of hormonal contraceptives, including IUDs, due to the hormonal effects on the endometrium.
Infection or inflammation: Possible causes of irregular bleeding, especially if the patient has recently had unprotected sex or has other symptoms like pelvic pain or discharge.
Polycystic Ovary Syndrome (PCOS): Can cause irregular menstrual bleeding, but would typically be associated with other symptoms such as acne, hirsutism, or weight changes.

Ectopic pregnancy: Although the patient has an IUD, the failure rate is not zero, and any pregnancy should be ruled out, especially if there's a possibility of the IUD being dislodged or if the patient has had unprotected sex.
Malignancy (e.g., cervical or endometrial cancer): Unlikely in a young patient but should be considered if the bleeding is persistent, heavy, or accompanied by other alarming symptoms like significant pelvic pain or systemic symptoms.
Pregnancy complications with the IUD in place: If the patient is pregnant, the presence of an IUD increases the risk of miscarriage, preterm labor, and other complications.

Thyroid dysfunction: Both hyperthyroidism and hypothyroidism can cause menstrual irregularities, including changes in bleeding patterns.
Coagulopathy: Bleeding disorders such as von Willebrand disease could present with irregular or heavy menstrual bleeding.
Adenomyosis or uterine fibroids: Less common in young women but could cause irregular bleeding, especially if large enough to distort the uterine cavity.