To answer several posts at once, aimed at the opening post.
Dysthymia is a term that although used again and again throughout the NHS isn't really used in the UK.
Referring to what one poster referred to. 'Endogenous' and 'Exogenous' depression were terms used in the past that are still used in some mental health circles, but it's really frowned upon and not cool. It was a widely held view some decades ago so still trickles down to today. Many people view it (the fully explained version rather than soundbites) as unhelpful or even offensive and view it as stuck in the 1970s and 1980s. If you want to look that up then google all that. Others see the earlier version of what it used to be called as an interesting way of explaining how society has seen depression (my view).