Many mini's overweight status goes way beyond counting calories by reducing volumes of feed and hay provided. THyroid is just one factor -- metabolic issues are normally the key player here. Your horse may indeed be metabolically challenged and require not only a reduction in feed/calories, but a restructuring of feed, reducing the nonstructural carbohydrates (NSC's, which equal starch and sugar) and fats.
I know that the Yahoo Cushing's site is rather daunting to read and sort through, but even if you join just for the FILES section, it would be educational. Another Yahoo list, The Metabolic Horse, takes a little more relaxed approach to the same set of issues and problems. Both are a worthwhile read:
Equine Cushings Group
Metabolic Horse Group
To keep it simple, if your horse is not already on a reduced starch/sugar diet (removal of all sweet feeds and/or grains, plus low sugar hay and/or soaking of hay), then this would be my first suggestion. Also, many minis will test normal on thyroid tests but may benefit from Thyro-L at any rate, simply from its effect of boosting the basal metabolic rate.
Bottom line is that reducing a high NSC feed in volume may not be enough in a horse that is highly sensitive to starch and sugar. A diligent effort at reducing the type of carbohydrates fed does not need to result in starving your horse. In fact, with the right type of food, you may actually be able to feed your horse MORE -- which will make your mini very happy I'm sure! :bgrin