I am having the same issue too. I posted today and noticed your post. I can't lose weight and only gaining and gaining fast. I joined kick boxing etc and I am so upset about this. Can it be hormonal?
I do not know much about leptin production. However, I do not think the uterus or ovaries produce leptin. Weight gain after hysterectomy is common but I do not know the mechanism. I personally do not have the energy, motivation, vibrancy, love of life I had prior to my hysterectomy so I am not as active. However, I have not gained weight because I do not have an appetite. I have also had diarrhea since my hysterectomy (which was severe for almost a year) so I lost a bit of weight before gaining it back. But I have maintained ever since (7 years).
Food does not have as much flavor except for sweets. I also no longer have the satiety sensation (satiety is associated with leptin). So if I did have an appetite, I would probably tend to overeat since I do not get that "fullness" feeling.