The hypothalamic-pituitary-adrenocortical axis in major depression.

The clinical utility of the tests of hypothalamic-pituitary-adrenocortical function discussed in this article are still to be ascertained. As with the dexamethasone suppression test, reviewed elsewhere in this issue, the application of other basal and challenge tests have furthered our understanding of the character of observed hypothalamic-pituitary-adrenocortical function in psychiatric patients. Hypothalamic-pituitary-adrenocortical physiology is complex and plastic and cannot be understood in terms of oversimplified explanations or concepts. Repeated measures over time, involving multiple assessments of axis activity, are essential in understanding the modulating effects of prior activity of the system (induced by endogenous regulatory systems or by superimposed environmental stressors) on the current function of the various levels in this closed-loop axis, where all parts influence the function of the remainder. From such an appreciation and understanding of this system can come more simplified assessments of its function, which would allow application to clinical psychiatry.