Background: Aromatase inhibitors (AIs), when used as adjuvant treatment of hormone receptor positive breast cancer, have been associated with bone density loss. However, any impact on PTH and by extension calcium (Ca) homeostasis is yet to be elucidated as no study till date has investigated longitudinal trends in parathyroid hormone (PTH) while on AI therapy. We explored these to determine if long term AI therapy was associated with hyperparathyroidism. Methods: This is a retrospective study of female breast cancer patients at the Cleveland Clinic Foundation started on AIs (anastrozole, letrozole and exemestane) from January 2015 to April 2022. Included patients had both baseline and subsequent data (after starting AI therapy), on serum Ca, PTH and lumbar spine T-scores (LsTs). These values were compared using paired t-test. Results: We identified n=9657 patients on AI therapy and included n=249 with pre and post AI PTH values. Among these, n= 209 received anastrozole, n=34 received letrozole and n=6 received exemestane. Cohort median age was 68 (IQR 62-72) years and median duration of AI therapy was 3 years. While we observed a significant positive correlation between change in Ca and change in PTH (p=0.02), the differences between baseline and treatment median serum Ca and PTH were not statistically significant. There was no significant association between changes in either serum Ca or PTH and change in LsTs (p-value: 0.69 and p-value: 0.05 respectively). Notably, 4% patients had elevated PTH and Ca after AI therapy was initiated among whom n=4 had markedly elevated PTH with low urinary Ca. Three of these four patients also had a diagnosis of chronic kidney disease (CKD) with normal vitamin D levels. Two patients with elevated PTH were on biotin supplements. There was an overall downtrend in LsTs (p-value: 0.0039) with no significant difference between the three AIs (p-value 0.0613). However, in comparison to other AIs, letrozole had the least depreciation in LsTs. Conclusions: Our findings do not suggest any correlation between AI therapy and hyperparathyroidism among breast cancer survivors. No significant increase in PTH levels occurred during AI therapy and majority of patients with clinically significant PTH elevation also had CKD, which may have contributed also to elevation of PTH level and low urinary calcium excretion.