The search for evidence of biological indices has up to now been focused on the kinds of markers that have been studied in depressed adults, such as abnormalities of cortisol physiology (Casat & Powell, 1988), melatonin (Shafii et al., 1996), thyroid-hormone levels (Dorn et al., 1996; Kutcher et al., 1991), sleep (Emslie et al., 1987), and brain imaging (Steingard et al., 1996). Several studies have shown that, in comparison with non-depressed patients, depressed young people are less likely to show suppression of cortisol secretion when the exogenous corticosteroid dexamethasone is administered (Casat & Powell, 1988) and more likely to have sleep abnormalities (Appelboom-Fondu et al., 1988; Cashman et al., 1986; Emslie et al., 1987; Kutcher et al., 1992; Lahmeyer et al., 1983; Riemann & Schmidt, 1993).
There has been very little longitudinal research on most of these measures. There is, however, some evidence that cortisol levels predict subsequent depression. Goodyer and colleagues (Goodyer et al., 1998) found that higher cortisol/DHEA levels at night predicted both the persistence of major depression and subsequent disappointing life events. They hypothesized that adrenal steroids might be involved in abnormal cognitive or emotional processes associated with the continuation of disturbed interpersonal behaviour. Susman et al. (1997) reported that adolescents who showed increased cortisol levels in a challenging situation had higher levels of depressive symptoms a year later than adolescents whose cortisol did not change or decreased.
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