Biological markers of affective disorders have not been studied intensively in patients with secondary depression. An elderly woman with severe delusional depression secondary to thyrotoxicosis was monitored with weekly dexamethasone suppression tests (DSTs) and three sleep electroencephalography (EEG) evaluations. She received treatment only for her thyrotoxicosis, but her depression resolved completely. The serial DSTs were normal throughout her depression, consistent with the specificity of this test for primary endogenous depression. The sleep EEG erroneously suggested a diagnosis of primary depression but effectively monitored clinical improvement. Biological markers may have applicability in evaluating and monitoring patients with secondary depression.