The most appropriate diagnostic test to perform next is a transvaginal ultrasound. This patient has likely developed Asherman syndrome (AS). Because the hormonal evaluation in this patient supports an intact hypothalamic-pituitary-ovarian axis, a structural abnormality such as AS should be suspected. AS is an uncommon complication of dilatation and curettage, intrauterine device placement, or surgical procedures such as hysteroscopic myomectomy; it is caused by lack of basal endometrium proliferation and formation of adhesions (synechiae). Diagnosis should be considered in any woman with amenorrhea and previous exposure to uterine instrumentation. The typical presentation is with secondary amenorrhea, at times associated with cyclic pelvic pain, created by distention of the uterine cavity where pockets of functional endometrium persist but efflux of menstrual flow is blocked or slowed by adhesion formation. Although some patients with AS may be completely amenorrheic, others may demonstrate hypomenorrhea and report scant menses compared with the volume of their menstrual flow before the procedure. AS most commonly occurs in an inflammatory setting such as endometritis or septic abortion. AS may also occur as a result of an overly aggressive curettage. In a patient with AS, transvaginal ultrasound will show a thin endometrial stripe and may reveal small pockets of fluid where menstrual flow has been trapped by neighboring adhesions. A functional uterine examination, such as hysterosalpingogram or saline sonohysterogram, confirms the diagnosis. Treatment consists of hysteroscopic resection of lesions.

Given this patient's normal gonadotropin levels, a pituitary cause for her secondary amenorrhea is unlikely; therefore, imaging of the pituitary is not warranted at this time.

Premature ovarian insufficiency is not the most likely diagnosis given this patient's normal estradiol and gonadotropin levels; therefore, a peripheral karyotype would be expected to be normal and should not be performed.

Results of a progestin withdrawal test are used to delineate between an estrogen-deficient state (no bleeding) and an estrogen-sufficient state (withdrawal bleeding). If the patient is producing estrogen, she will have withdrawal bleeding within 1 week of completing a course of progesterone. If no withdrawal bleeding occurs after the progesterone challenge, then the patient has either a low-estrogen state and hypothalamic amenorrhea is the diagnosis, or there is uterine outflow blockage. This patient's history of a previous uterine procedure prior to the onset of amenorrhea and the possibility of denuded endometrium where synechiae are present will make a negative test (no withdrawal bleeding) uninterpretable.