What does losing weight have to do with hormones? Everything! Well, that's a slight exagerration, but don't let anyone tell you a calorie is a calorie. Hormones can make or break almost any weight loss program as I'll show below.
The problem is this: if you are young with ample hormonal levels, weight loss is straighforward: a little exercise and reduction in calories and the pounds melt off. How many of us in middle age and beyond, though, have cut back in calories and increased exercise only to find not much happened?
One of the reasons for this is that so many of us in modern, industrialized societies have hormonal deficiencies. The power of hormones over our health and weight maintenance cannot be overestimated and the studies show this conclusively.
This is a big subject, of course, but below we outline some of the key studies that show the intimate link between certain hormones and the body's response to dieting.
1. Testosterone. One study of senior men looked at changes in body composition after medium term (3 year) testosterone administration. These men lost nearly a kg per year, or 2.2 pounds, in fat mass just from the testosterone.  In other words, after three years these men had lost on average about seven pounds of fat without modifying their lifestyle. Furthermore, these were not hypogonadal men necessarily, where testosterone administration would have produced even more pronouned results.
In hypogonadal men, i.e. if your testosterone is considered to be clinically low, results can be dramatic. For example, one study of such men found that average fat loss was 14%!  Again, this is with no other change in lifestyle - nothing except letting this one hormone do its work. (Of course, muscle mass increased substantially as well.) An earlier study showed that there was a dose dependent increase in body composition. In other words, the more the additional testosterone given, the greater the change in lean body mass. 
So the bottom line with testosterone is that weight loss can be substantial, depending on your baseline testosterone before and after treatment.
2. Growth Hormone. Men rapidly lose growth hormone with aging and this hormone can have just as significant of an impact on losing weight as testosterone. For example, one study in senior men showed a 15% drop in body fat from administration of growth hormone.  That's a lot of fat to lose with no other real lifestyle change accompanying it.
Admittedly, the bodies of these men had very little natural growth hormone production, but many other studies have showed significant positive change in body fat and muscle from administration of growth hormone.  See my link on How to Increase Growth Hormone Naturally for additional information.
NOTE: Growth hormone will likely have little affect on young, health males with strong growth hormone production. 
3. Thyroid. "It's got to be my thyroid." We've all heard that excuse, eh? Well, of course, it is true that an underactive thyroid can lead to significant weight gain. A slow thyroid means a slow metabolism and to study how much of an effect thyroid function has on weight gain, reseearchers recently examined patients in the well-known Framingham Offspring Study.
They grouped participants into quartiles of increasing TSH (Thyroid Stimulating Hormone), which is a strong predictor of thyroid function. In general, if you have high TSH, it means you have an underactive thyroid. Researchers found that in men, there was a almost a three pound difference in body weight from the lowest to highest TSH quartiles over a 3.5 year period. The authors noted that even "modest changes" in TSH could result in weight gain.
Of course, it's possible to be deficient in one or more of these above three hormones, which only compounds the problem. Anyone at or past middle age and experiencng weight gain may want to consider going to an endocrinologist or other knowledgeable physician for a brief workup. If you do have a hormonal issue, it's something you want to know about.
NOTE: Appetite is controlled by other key hormones, which I discuss in my link on Weight Loss and Appetite Hormones.
1. Sleep. Lack of sleep will lower both testosterone and growth hormone. (Apnea sufferers can find their testosterone dropping by up to 40 percent for example.) See my link on percent for example.) See my link on Sleep and the Appetite Hormones as well: poor sleep virtually forces us to overeat.
2. Vitamin D. This quasi-hormone has recently been discovered to have a profound impact on both testosterone and thyroid function, particularly if deficiencies are present. See my link on Vitamin D for more detail.
3. Cortisol. This stress hormone can affect both testosterone and thyroid function negatively. In addition, cortisol is famous for increasing belly, or visceral fat. If that spare tire inflates rapidly, you may want to get your cortisol levels checked. For more information on the cortisol-visceral fat connection, see my link on Visceral Fat.
SOBERING FACT: All of this can create a vicious circle: put on enough extra weight and it can significantly lower both testosterone and growth hormone.
The bottom line: untreated medical conditions, poor lifestyle choices and ignorance can lead to decreased hormone levels, which in turn make weight management and loss much more difficult.
1) The Journal of Clinical Endocrinology & Metabolism, Aug 1 1999, 84(8):2647-2653, "Effect of Testosterone Treatment on Body Composition and Muscle Strength in Men Over 65 Years of Age"
2) The Journal of Clinical Endocrinology & Metabolism, Dec 1 1996, 81(12):4358-4365, "Increase in bone density and lean body mass during testosterone administration in men with acquired hypogonadism"
3) Metabolism, Jun 1985, 34(6):571-573, "The effect of anabolic steroids on lean body mass: The dose response curve"
4) Hormone Res in Pediatrics, 1991, 36(Suppl. 1):73-81"Effects of Human Growth Hormone on Body Composition in Elderly Men"
5) Clinical Endocrinology, Jul 1992, 37(1):79ï¿½87, July 1992"The effect of growth hormone administration in growth hormone deficient adults on bone, protein, carbohydrate and lipid homeostasis, as well as on body composition"
6) Annals of Internal Med, Dec 1 1996, 125(11):883-890, "Effects of Physiologic Growth Hormone Therapy on Bone Density and Body Composition in Patients with Adult-Onset Growth Hormone Deficiency"
7) Acta Endocrinol, Apr 1 1993, 128 313-318, "Effect of growth hormone treatment on hormonal parameters, body composition and strength in athletes"
8) Archives of Internal Med, Mar 24 2008, 168(6):587-592, "Relations of Thyroid Function to Body Weight"