Views about the nature of depression have changed over time but the debate over types of depression and the ways they might be classified continue. There remains no specific diagnostic test for depression beyond questionnaires and the reliance on self-reports from sufferers and those close to them. What remains are clusters of symptoms and it isn't clear whether these point to one or more diseases.
When I started out in psychiatry the classification of depression was different. We had what was known as endogenous or reactive types. This was based on the notion that depression might occur as a result of some internal (endogenous) state that was most likely to be genetic in origin. Reactive depression, by contrast, was thought to be due to some level of severe stress and the sufferer would show very marked symptoms of worry and anxiety. What's wrong with these ideas is the fact we know people with so-called endogenous depression were as likely to suffer stress as people with reactive depression. Furthermore, there is no greater success rate with antidepressants in either case and people with endogenous depression are as likely to be worried and anxious as those with reactive depression.
Back to the drawing board. Well, not quite. There is still a view that depression can be inherited but to varying degrees. Whether depression is seen may then depend upon the severity of stress in the person's life to trigger an event.
These days the medical profession is more likely to think of depression in terms of whether it is primary or secondary. In primary, the depression is viewed as having developed outside of another medical cause. In secondary, the depression follows a medical or psychological condition. And there are quite a few conditions and diseases known to be the precursor of depression. For example, hypothyroidism, Parkinson's Disease, AIDS, schizophrenia, Alzheimer's disease, bulimia, panic disorder, stroke, or heart disease. It's not so long ago that depression was seen as an unfortunate side-effect of such conditions and was often left untreated.
There's no real evidence to suggest there is any difference between primary or secondary depression. The issue is really one of classification and an acknowledgement of cause. It helps to know that by treating certain physical complaints the chance of recovery from depression is increased. We also know that people suffering with secondary depression tend to have less severe and less life-threatening forms of depression. In either case there is at least a recognition that depression needs to be treated in its own right no matter what the cause.