Sex Differences in Risk for Iatrogenic Thyrotoxicosis Among Older Adults: An Analysis from Real-World Clinical Data

Background: Overtreatment with thyroid hormone is common and is associated with multiple adverse health outcomes, especially in older adults. Higher overtreatment rates have been reported among women. Understanding this sex difference could lead to better clinical utilization of therapy in at-risk populations. Methods: We performed a retrospective cohort study to examine the relationship between iatrogenic thyrotoxicosis and patient sex, adjusting for demographics, comorbidities, body composition, and thyroid hormone dosing using electronic health records for adults age 50 and older in a large community health system in the United States. Results: A total of 20,724 patients met all inclusion criteria, of whom 77% were female and 23% non-White. Women were more likely to have a low thyrotropin (TSH) compared to men (36.7% vs. 23.9%; unadjusted hazard ratio [HR] = 1.67 [95% confidence interval {CI} 1.56-1.79]). Many covariates varied by sex, including rates of several comorbidities, and there was a small but statistically significant difference in the median daily levothyroxine dose per actual body mass: 1.1 μg/kg in male patients compared with 1.2 μg/kg in female patients (p < 0.001). Adjusting for covariates other than dose did not significantly change the sex-related risk of iatrogenic thyrotoxicosis. In fully adjusted Cox models including dose per actual body mass, the female versus male HR = 1.50 [CI 1.34-1.69] was also not different from models which did not account for dose (p = 0.422). However, the association between female sex and thyrotoxicosis risk was partially mediated when adjusting for dose per ideal body mass (HR = 1.30 [CI: 1.16-1.46]; p < 0.001) and was fully mediated by dose calculated using lean body mass (HR = 1.06 [CI: 0.95-1.19]; p < 0.001). That is, women had higher risk of iatrogenic thyrotoxicosis compared to men receiving similar actual body mass doses, whereas women and men receiving comparable lean body mass doses had comparable risk. Conclusions: Mediation analysis demonstrates that the increased risk of iatrogenic thyrotoxicosis in older women may be attributable to differences in body composition between men and women. The use of ideal or lean body weight-based dose calculators in place of actual body weight dosing could reduce this potential source of iatrogenic thyrotoxicosis risk in older women.