Dear ritaav, The recent study regarding the switch from tamoxifen to letrozole was in women that had completed 5 years of tamoxifen treatment for their breast cancer. Current data does not support the switch to letrozole prior to completing the full five years of tamoxifen therapy, although their are clinical trials exploring this question.
The levels that would particularly be evaluated would be the FSH, LH, and estradiol levels. There is not a specific number that is looked at but more the ratio of FSH to LH. Research suggests that a high serum FSH to LH ratio (1.9-3.8) may be a useful indicator of menopausal status. The above levels are evaluated along with physical changes and age to determine a person's menopausal status.