How do you manage endocrine therapy for a perimenopausal woman with ER+ breast CA?

In lower risk patients to whom I would not routinely give chemotherapy - stage I-IIA, with a low or low-intermediate risk Oncotype score - I start tamoxifen and switch to an aromatase inhibitor (except in very low risk patients in whom the benefit of switching would be negligible) when they are clinically (no menses >12 months) and biochemically (by FSH and estradiol levels) postmenopausal. In higher risk patients to whom I routinely administer adjuvant chemotherapy, I expect most will be permanently postmenopausal after the chemotherapy, so I start with an aromatase inhibitor and monitor (clinically and with serial estradiol levels) for recovery of ovarian function; if ovarian function recovers, I recommend adding ovarian suppression or ab...