I suspect I've had lowish testosterone all my life, though I didn't start getting numbers on T until a year and a half ago. And I've had mild anxiety (in particular, social anxiety and lack of self-confidence) for most of my life. I couldn't begin to say whether one cased the other.
Like most people, I've had waxing and waning "real" reasons to have anxiety over the years, and at many points, there were external factors that could have reasonably made me anxious (or depressed) but which did not necessarily give me significant trouble "carrying on." My feeling is that something qualitatively different started happening 3-4 years ago---for example, I developed a fear of air travel, having flown with pleasure for 25 years previously. Similarly with increasing anxiety about being far from home, or about being in confined spaces from which I couldn't immediately escape. And I grew increasingly overwhelmed, intellectually and emotionally, by tasks at work which previously I'd embraced. Then when life again really gave me things to be anxious about in the last half year, I fell apart emotionally---frequent, physical symptoms of anxiety, perhaps bordering on panic disorder.
Unfortunately, I don't have very good T-level information to tie to this. I was tested for the first time in Spring 2014 and was on the low end of normal (460). In the midst of major stress and after months of repeated respiratory infections, in Spring 2015, three tests consistently put me in mid-upper 200s.
Then I developed libido/ED problems in May (which of course doesn't help with anxiety!) but mercifully started TRT (compounded cream) in early July. It's hasn't solved all my problems (just about lost my mind being stuck in an hours-long traffic jam a few weeks ago), and we're still adjusting dosages to get beyond the quite modest improvements in T levels, but the anxiety / sense-of-well-being situation is much improved. I remember, just a week into TRT, being in a couple of situations where my mind told me, "This is the sort of situation that generates anxiety/panic in you," but my body simply wasn't producing anxious feelings. It was uncanny.
In terms of medication, since the spring, I have occasional use of Klonopin (half of a .5 mg tablet) to help with sleep, maybe once every week or two. I have no idea whether this has had an impact on testosterone levels or ED/libido. Acknowledging the many posts about the dangers of benzos, I have to say that this occasional use is very helpful for anxiety.
I'm particularly curious whether other posters got "early warning" of Low T via depression/anxiety, ahead of more obvious sexual issues like low libido or ED.