Biochemical Profile at Residential Admission in Anorexia Nervosa: Comparable Findings Across Restricting and Binge–Purge Subtypes

Background: Anorexia nervosa (AN) is associated with extensive biochemical alterations at the time of clinical presentation. Despite distinct behavioural profiles, it remains unclear whether the restricting (AN-R) and binge–purge (AN-BP) subtypes differ in their biochemical profile at residential admission, a question with direct implications for clinical monitoring protocols. Methods: We conducted a retrospective cross-sectional study of 177 consecutive AN inpatients (AN-R: n = 130; AN-BP: n = 47; 98.3% female; mean BMI 15.8 ± 2.0 kg/m2; mean age 19.2 ± 5.5 years; 53.7% adolescents) admitted to a specialist residential unit (2018–2026). Thirty-five biochemical parameters were assessed. Unadjusted comparisons used Mann–Whitney U tests with rank-biserial effect sizes; multivariable logistic regression adjusted for BMI, age, and illness duration. Results: Overall, 98.3% of patients presented at least one biochemical anomaly (mean 4.4 ± 2.6 per patient). The most frequently abnormal parameters were LDH elevation (75.0%), vitamin D insufficiency (73.0%), low T3 syndrome (defined as low free triiodothyronine [FT3]) (58.0%), FT4 suppression (53.5%), ALT elevation (35.1%), and leucopenia (32.3%). AN subtype predicted none of these outcomes in unadjusted or multivariable analyses (all p > 0.10). The odds of presenting ≥3 simultaneous anomalies were not significantly associated with subtype, BMI, or illness duration in multivariable analysis. In contrast, older age independently predicted greater odds of low T3 syndrome (OR = 1.14 per year, 95% CI [1.00, 1.30], p = 0.045) and leucopenia (OR = 1.22 [1.06, 1.39], p = 0.004). No differences were observed between adolescents and adults in unadjusted comparisons. Conclusions: Despite their distinct behavioural profiles, no statistically significant biochemical differences were detected between AN-R and AN-BP at residential admission. These findings support routine standardised biochemical screening in all patients with anorexia nervosa admitted to residential care, regardless of behavioural subtype. Older age may identify a subgroup warranting closer endocrine and haematological monitoring.