Differentiating constitutional thinness from anorexia nervosa in DSM 5 era

•How can constitutional thinness (CT) be distinguished from anorexia nervosa (AN) after amenorrhea removal in DSM5 definition?.•For the 150 subjects, only DEBQ Restrained Eating subscale was successful to differentiate AN from CT.•Free-T3 and Leptin were strong significant tools for AN and CT distinction with high sensitivity and specificity values.•Psychological criteria should be completed by at least a free-T3 assay to differentiate CT from AN.•Distinction between CT and AN will help avoiding social stigmatization and excessive useless therapies for CT women. Constitutional thinness (CT) is an underweight state characterized by normal menstruations and no change in feeding behaviour. Thinness is the only resemblance between Anorexia Nervosa (AN) and CT. Removal of amenorrhea from the new DSM 5 definition of AN might result in misdiagnosis between these two populations. The objective of this study was to compare CT, AN and Control subjects in terms of biological, anthropometric, and psychological markers in order to better distinguish AN from CT subjects. Body composition, nutritional markers, pituitary hormones, bone markers and psychological scores were evaluated in three groups of young women: fifty-six CT, forty restrictive-type AN and fifty-four Control subjects. For every marker, a receiver Operator Characteristics (ROC) curve was calculated to evaluate the accuracy of differentiation between AN and CT groups. For most studied parameters, CT subjects were similar to Controls but dramatically different from AN subjects. DEBQ Restrained Eating subscale score was identified by ROC data analysis as the only psychological parameter tested to successfully differentiate AN from CT. Free-T3 and Leptin were shown to be powerful markers to differentiate AN and CT populations as they were highly specific and sensitive ones. The exclusive use of psychological testing criteria is not always sufficient to differentiate AN and CT patients. Minimally, additional testing of Free T3 levels, which is cheap and widely accessible for general practitioners, should be completed to avoid misdiagnosis which could result in the implementation of ineffective treatment plans and social stigmatization for CT women.