Estradiol Levels According to Body Mass Index in Premenopausal Patients With Breast Cancer

his prospective observational study will evaluate whether body mass index (BMI) is associated with serum estradiol levels in premenopausal women with hormone receptor-positive breast cancer. Participants will already be receiving standard adjuvant treatment with an aromatase inhibitor and pharmacologic ovarian suppression as part of their routine clinical care. The study will not assign or modify cancer treatment. Approximately 172 participants will be enrolled at multiple study sites in Brazil. They will be classified into two main groups: normal BMI and overweight or obesity. Clinical information, medication use, physical activity, weight, height, and serum estradiol levels will be recorded. Estradiol will be measured using an ultrasensitive laboratory method, with the main assessment occurring after approximately three months of treatment. The primary objective is to compare the proportion of participants with serum estradiol levels above 2.72 pg/mL between the two BMI groups. The study will also examine the relationship between BMI and absolute estradiol levels and explore whether clinical characteristics, previous chemotherapy, physical activity, or use of glucagon-like peptide-1 receptor agonists are associated with inadequate estrogen suppression.

Inclusion Criteria: Female sex. Age 18 years or older. Histologically confirmed hormone receptor-positive invasive breast cancer, defined as estrogen receptor-positive and/or progesterone receptor-positive disease, regardless of human epidermal growth factor receptor 2 status. Premenopausal status at the initiation of ovarian suppression, as determined by menstrual history and/or laboratory criteria according to medical assessment. Indication for adjuvant treatment with an aromatase inhibitor (anastrozole, letrozole, or exemestane) combined with pharmacologic ovarian suppression using a gonadotropin-releasing hormone analog. Continuous use of an aromatase inhibitor and a gonadotropin-releasing hormone analog for at least 3 months before the primary study assessment. Documented body mass index at enrollment. Ability to understand the study information and provide written informed consent. Exclusion Criteria: Postmenopausal status at the initiation of endocrine therapy. Known endocrine disorders that may interfere with the production or metabolism of sex steroids. Concomitant use of medications known to interfere with estradiol metabolism or measurement, including systemic corticosteroids at supraphysiologic doses. Metastatic disease at enrollment. Technical or logistical inability to undergo serum estradiol measurement using an ultrasensitive laboratory method. Concurrent participation in another clinical study that may interfere with the hormonal outcomes evaluated in this study.