Low Testosterone and depression. Unfortunately, the two seem to be married at the hip. The reason is simple: depression lowers testosterone and low testosterone leads to depression.
Many men simply do not realize that testosterone dramatically affects the male brain. Sure, testosterone is the "sex hormone" and the "muscle hormone". But I would argue that even more impressive is its ability to affect mood and cognition. As testosterone decreases, anxiety, mental fog and depression increase until, eventually, it becomes hard to function in marriages and careers. And that isn't good for anyone.
I have to admit that I was surprised often the subject of depression came up when I started the Peak Testosterone Forum. Perhaps I shouldn't have been, but it was simply stunning how many of the posters complained of depression either currently or recently and the usage of various antidepressants.
"I was wondering if anyone knew anything about 5-HTP? Is it a good thing to take? I was thinking of taking it for depression. I have slumped into a depressive state after my last job placement came to an end. I feel quite hopeless, a failure" 
"I suffer from many of the publicised symptoms along with a long spell of depression and feel that I need to address this testosterone level issue to address this and the other symptoms. " 
"Hi there all.i am new here.been on androgel since 2/16/12..was hypogonadal. The psychological part started when I thought that was it for me...it was then of course I started feeling down and not knowing anything about depression and anxiety and what it was actually doing to my body at the time ( T levels etc ) then just getting steadily worse further down the line" 
"People were noticing at work that I wasn't cracking the stupid jokes I used to dish out. I started to go into >a slowly ,accelerating depression until the age of 30-31 when it became full blown major depression with crying fits that would make a grandmother look like Chuck Norris in comparison to myself." 
"This decline has happened over the last 3 years max. how can i differentiate between depression and something wrong with hormones?" 
"I have talked with the Dr. about the depression but I cannot take anti-depressants ( am in the military they do not like people taking them)." 
These posts could go on and on and are tragic to say the least and I have not even included the many discussions about Wellbutrin, Paxil, SSRI's and so on. You may want to read this Thread: Roll Call on Depression for some interesting reads as well.
As you can see, usually the men believe that over time lower and lower testosterone levels lead to an increased state of depression. Generally, though not always, they do not attribute their depression as actually causing the low testosterone. So that leads to a "chicken and egg" question: which usually comes first?
Of course, showing "causality" as it is called is tricky business. However, we can tease out some interesting conclusions from this simple fact: HRT (testosterone therapy) has many studies showing that it dramatically improves depression and mood. Just look at some of the studies that show just this:
NOTE: Although I am focusing on the opposite, studies definitely show that depression can lower testosterone. This is no surprise since depression raises cortisol and cortisol fight testosterone. For example, one study found that daytime and nightime testosterone levels were lower in men with depression and the trigger molecule to the testes, leutinizing hormone, was decreased as well.  And, as one would expect, one study found that the greater the depression, the lower the testosterone levels independent of age. (See my link on Natural Depression Cures for possible helps.)
1. Refractory Depression. Refractory depression is depression that does not respond to standard treatments. Interestingly enough, one small study found that "testosterone gel may produce antidepressant effects in the large and probably underrecognized population of depressed men with low testosterone levels."  Of course, this implies that in many hard-to-treat men low testosterone is actually a root cause. Tragically, many doctors never even think to do a blood draw.
2. Standard Depression. One meta-analysis using 7 studies found the same thing: testosterone therapy exerted a significant anti-depressant effect on patients with existing depression. 
3. Hypogonadal Men. One would not necessarily expect testosterone supplementation to help men with normal testosterone, healthy testosterone levels. However, one would expect that if men had low testosterone, Hormone Replacement Therapy might give them a nice boost in mood parameters. This is exactly what one study found.  One study with Testim found that only was mood improved but also body composition, i.e. more muscle and less fat.  This common with testosterone therapy and will help your depression, undoubtedly, due to improved self-image as well.
4. HIV Infected Men One study showed that men with AIDs may experience a significant boost in depression from testosterone therapy. [17}
5. Seniors. Certain groups of senior men may benefit from HRT as well.  This is not too suprising since testosterone decreases with aging. See my links on The Causes of Andropause and Hormone Replacement Therapy for more information.
So there are clearly man subpopulations of men where testosterone can significantly help with depression. This is, of course, something to discuss with your physician if it applies.
How long does it take for the improvements in depression to hit? One study found that the maximum improvement in mood occurred at about 30 days and then no significant further improvements occurred.  However, this may or not be the case for you. Some men feel the difference almost immediately and others can take several months.
NOTE: Testosterone may actually help the women in your life with depression in some cases as well and several studies have shown just that. Of course, women have lower seratonin levels and, therefore, tend to struggle more with depression than men. However, even though their testosterone levels are about a tenth of ours, it looks like they may benefit in some cases as well.
8) Am J Psychiatry, Jan 2003, 160:105-111, "Testosterone Gel Supplementation for Men With Refractory Depression: A Randomized, Placebo-Controlled Trial"
9) Journal of Psychiatric Practice, Jul 2009, 14(4):289-305, "Testosterone and Depression: Systematic Review and Meta-Analysis"
10) J Clin Endocrinol Metab, 1996, 81:3578 3583, "Testosterone replacement therapy improves mood in hypogonadal men a clinical research center study"
11) BJU Int 91(1):69-74, "A novel testosterone gel formulation normalizes androgen levels in hypogonadal men, with improvements in body composition and sexual function"
12) J Clin Endocrinol Metab , 2000, 85(8):2839-2853, "Transdermal testosterone gel improves sexual function, mood, muscle strength, and body composition parameters in hypogonadal men"
13) Maturitas, 15 April 2002,, 41(Supplement 1):25-46, "The impact of testosterone imbalance on depression and women's health"
14) Menopause, Sep 2003, 10(5):390-398, "Transdermal testosterone therapy improves well-being, mood, and sexual function in premenopausal women"
15) The Journal of Clinical Endocrinology & Metabolism, Feb 1 1999, 84(2):573-577, "Bioavailable Testosterone and Depressed Mood in Older Men: The Rancho Bernardo Study"
16) The Journal of Clinical Endocrinology & Metabolism, Jan 1 2000, 85(1):60-65, "Effects of Hypogonadism and Testosterone Administration on Depression Indices in HIV-Infected Men"
17) Biological Psychiatry, Feb 1985, 20(2):222-225, "Plasma testosterone levels, depression, sexuality, and age"
18) Journal of Clinical Psychiatry, Dec 2002, 63(12):1096-1101, "Testosterone therapy in late-life major depression in males"