Constitutional thinness may be difficult to distinguish from anorexia nervosa. A number of laboratory tests can help in making the differential diagnosis.
Both constitutional thinness and anorexia nervosa have a low BMI (< 18.5 kg per square meter).
A woman with constitutional thinness typically has normal menstruations and feeding behaviors with no resistance to body weight gain (and may welcome it).
Laboratory tests:
(1) free T3: a free T3 < 3.3 pmol/L indicates anorexia nervosa; normal in constitutional thinness
(2) leptin: very low in anorexia nervosa; in constitutional thinness it is lower than normal controls
(3) ghrelin: high in anorexia nervosa; low in constitutional thinness
(4) estradiol: low in anorexia nervosa but affected by oral contraceptives
The circadian variation of leptin in constitutional thinness is similar to normal controls while it is blunted in anorexia nervosa.
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