Background: We investigated, in an exploratory cross-sectional design, whether EDS-R score at admission showed adjusted cross-sectional associations with FT3, LDH, and ALT in patients with anorexia nervosa (AN) in residential treatment. Methods: A total of 164 patients with AN (first admission per patient) (AN-R: n = 122; AN-BP: n = 42; mean BMI 15.7 ± 1.7 kg/m²; 98.2% female) admitted consecutively to a specialist residential unit were included. Biochemical parameters were assessed within 48 h of admission using fully automated, internationally standardised analytical methods. FT3 was measured by electrochemiluminescence immunoassay (ECLIA) on automated immunoassay platforms, in accordance with standard immunometric procedures. ALT and LDH were measured by kinetic enzymatic (UV) methods following IFCC recommendations, using high-throughput clinical chemistry analysers. All analyses were performed in the core laboratory (Corelab) of the ASL Salerno hospital network, with internal and external quality control programmes in place. Exercise dependence severity was measured using the EDS-R Multivariable OLS and logistic regression analyses were conducted, adjusting for age, BMI, disease duration, and AN subtype. Results: Among 16 biochemical parameters examined, FT3 and LDH showed the largest Spearman correlations with EDS-R score, though neither survived Bonferroni correction for 16 simultaneous tests (threshold: p < 0.0031) (FT3: ρ = −0.224, p = 0.007; LDH: ρ = +0.244, p = 0.006). In multivariable regression, EDS-R score was independently associated with lower FT3 (β = −0.040, 95% CI [−0.072, −0.008], p = 0.016, n = 144) and higher LDH (β = +14.56, 95% CI [3.78, 25.35], p = 0.009, n = 124), while BMI was not significant in either model. By contrast, BMI was the sole significant predictor of ALT (β = −0.078, 95% CI [−0.136, −0.021], p = 0.008, n = 154), with the EDS-R score being non-significant (β = 0.019, p = 0.604). Binary logistic regression confirmed an association between EDS-R score and low-T3 syndrome (FT3 < 2.30 pg/mL): OR = 1.290 (95% CI [1.043–1.596], p = 0.019). Conclusions: In this exploratory cross-sectional study, EDS-R score showed adjusted associations with lower FT3 and higher LDH, but not ALT, at residential admission in AN, independent of BMI, age, AN subtype, and illness duration. These hypothesis-generating, cross-sectional findings cannot be interpreted as establishing prediction, causation, biomarker validity, or clinical utility. Prospective replication is required before any clinical implications can be considered.