Biological classification of depressive illness.

1 A number of biological criteria have been used for refining clinical classification to identify patients who will respond to antidepressant treatment. 2 Subtypes of depressive illness have been postulated to occur depending on whether there is a relative deficiency of central noradrenaline or 5-hydroxytryptamine. However, antidepressants having selective effects on amine systems have similar therapeutic effects and inhibition of re-uptake of these amines does not correlate with clinical outcome. 3 An abnormal response to thyrotrophin-releasing hormone has not been confirmed as specific for depressive illness. Failure to control for age and sex may account for some of the discordant findings. 4 It has been suggested that depressed patients secrete less growth hormone to a variety of stimuli. It appears that the predictive value of a growth hormone response for the diagnosis of endogenous depression is 53%. 5 The dexamethasone suppression test appears to have high sensitivity for the diagnosis of endogenous depression but low specificity and should therefore be used only in conjunction with clinical assessments.