A few men on the Peak Testosterone Forum who have started HRT have reported increased appeitite and have wondered if the two are related. The answer, it turns out, is surprisingly involved. First of all, testosterone definitely affects the two most well-known appetite and hunger hormones, gherlin and leptin.
Let’s start with leptin. Leptin is proportional to body fat and, therefore the more overweight you are, the higher your leptin levels. And rising leptin levels slowly turn off your appetite, which, of course, is a good thing if you have put on some extra weight. However, the reverse situation can really cause men difficulty when dieting: as you lose weight, your leptin falls and hunger increases. This, of course, makes it significantly more difficult to stick with a diet (along with a dozen other nasty hormonal changes including plummeting testosterone levels that I discuss in my link on Why Crash Dieting Usually Does Not Work).
So how does testosterone tie into this? Simple: researchers have noted that leptin goes down if you put a man on testosterone. One study found that the “serum leptin concentration correlated inversely with that of testosterone in elderly men. This inverse correlation was still present when body mass index and plasma insulin were included in the analysis. The administration of testosterone to young men suppressed serum leptin from the pretreatment level of 3.4 Â± 1.4 to 1.9 Â± 0.6 Î¼g/L during the therapy. After cessation of testosterone injections, serum leptin concentration returned back to the pretreatment level.” 
Well, it turns out that is probably not the case and here is why: hypogonadal men tend to have overly high leptin levels and giving them testosterone merely brings them back to where they should be. One study stated that “we conclude that hypogonadal men exhibit elevated OB [prolactin] levels that are normalized by substitution with T.” 
And this is verified by the fact that many stuidies have shown that men who go on HRT (testosterone therapy) actually lose weight. In fact, they lose weight without making any other change except increasing testosterone. In addition, they gain muscle as well. So even if testosterone does increase appetite a little through leptin reduction, this is apparently more than compensated for by something else, including things like a) increasing a man’s desire to workout due to mood elevation and improved recovery, b) lower insulin levels and c) lower cortisol “reactivity.” (The latter two should help a guy better manage visceral fat.) Again, though, testosterone in hypogonadal men more than just returns them to their baseline levels.
Gherlin is the opposite of leptin in the sense that, as body fat increases, gherlin levels tend to decrease as shown by the fact that weight loss through dieting increases gherlin levels (in obese individuals).  Furthermore, ghrelin levels increase significantly right before a meal and drop shortly after. In other words, increasing gherlin means increasing appetite.
One study found that control-matched men has significantly lower gherlin levels and that giving them testosterone restored their gherlin levels to normal.  Now this may seem like a bad thing, because one would want gherlin as low as possible to suppress appetite, right? Well, not so fast. It turns out gherlin is VERY important for the brain and having adequate levels may be critical for protecting the all-important hippocampus. Researchers noted that
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“Here we report that circulating ghrelin enters the hippocampus and binds to neurons of the hippocampal formation, where it promotes dendritic spine synapse formation and generation of long-term potentiation. These ghrelin-induced synaptic changes are paralleled by enhanced spatial learning and memory. Targeted disruption of the gene that encodes ghrelin resulted in decreased numbers of spine synapses in the CA1 region and impaired performance of mice in behavioral memory testing, both of which were rapidly reversed by ghrelin administration.” 
In other words, artificially lowering gherlin is NOT a good idea and, once again, testosterone is protecting your brain. (See other examples in my page on Testosterone and the Brain.)
Thus, generally speaking, testosterone should not increase your appetite to unhealthy levels but rather just return them to your normal, youthful baseline levels. And I suspect that the reported increase in appetite from testosterone therapy is usually resulting from the longer, harder workouts from finally feeling good.
1) The Journal of Clinical Endocrinology & Metabolism, Sep 1 1998, 83(9):3243-3246, “Inverse Correlation between Serum Testosterone and Leptin in Men”
2) N Engl J Med, 2002 May 23, 346(21):1623-30, “Plasma ghrelin levels after diet-induced weight loss or gastric bypass surgery”
3) The Journal of Clinical Endocrinology & Metabolism, Aug 1 1997, 82(8):2510-2513, “Testosterone Substitution Normalizes Elevated Serum Leptin Levels in Hypogonadal Men”
4) Testosterone therapy J Clin Endocrinol Metab. 2003 Sep;88(9):4139-43, “Testosterone replacement therapy restores normal ghrelin in hypogonadal men”
5) Nature Neuroscience, 2006, 9:381 – 388, “Ghrelin controls hippocampal spine synapse density and memory performance”