Differential effects of teprotumumab treatment based on fat-to-muscle ratio in patients with thyroid eye disease

Purpose: To characterize the distribution of fat-to-muscle ratio (FMR) across patients with thyroid eye disease (TED) and to assess the association between FMR and therapeutic response to teprotumumab. Methods: A retrospective cohort study of patients completing a full course of teprotumumab for TED between January 2020 and March 2022 at a single tertiary referral center. Patients without baseline orbital imaging were excluded. Quantitative analysis of FMR was performed by manual segmentation of patients' imaging using OsiriX software. The primary outcome measure was change in clinical measurement of proptosis. Linear regression modelled change in proptosis against FMR. Statistical significance was set at p < .05. Results: Twenty-two patients (3 M:19F) were included with a mean age of 49.4 +/- 15.5 years. The FMR ranged from 1.11 to 6.54, mean 3.15 +/- 1.30. The data did not deviate from a normal distribution (Shapiro-Wilk test for normality, p = .18). Pre- and post-treatment average proptosis measurements were 21.72 +/- 3.56 mm and 18.81 +/- 3.07 mm, respectively. Univariable linear regression demonstrated a 0.78 +/- 0.36 mm greater reduction in proptosis for every 1 unit decrease in FMR (p = .038). Conclusions: Contrary to the traditional dichotomous characterization of TED into type 1 and type 2 phenotypes, orbital FMR may represent a continuum of disease manifestation, more closely following a normal rather than bimodal distribution. Furthermore, pre-treatment FMR is associated with response to teprotumumab; those with lower FMR experiencing a greater reduction in proptosis. This has implications for patient selection and counselling regarding the expected treatment outcome.