Would you consider switching endocrine therapies for a peri-menopausal woman on adjuvant tamoxifen solely due to an estradiol level if this was checked and found to be significantly elevated?

I agree with Dr. @Dr. First Last that this is likely ovarian hyperstimulation from tamoxifen which can occur in 9-17% of women and is more likely to occur in younger women treated with tamoxifen therapy alone. In one study, a level of over 300pg/mL and follicular diameter over 30mm were used to define OHS. (Yamazaki et al., PMID 31996163) I am not aware of any data regarding a specific estrogen cutoff that is used to force a switch in therapy because there isn't a lot of outcome data to say it is required. A study on largely tamoxifen treated women by the WHEL group did suggest worse outcomes for women with persistently elevated estrogen levels versus those that did not (avg. 22pg/mL vs 11pg/mL). (Rock et al., PMID 18323413) However, hormon...

Tamoxifen is a potent inducer of ovarian function. If on adjuvant tamoxifen and the estradiol level increased to that extent, I would consider adding an LHRH agonist (such as goserelin). Tamoxifen can be continued at that time or switched to an AI, depending on the patient's risk factors.