Hi Nicky,
About 20-30 percent of patients with Graves' disease gain weight. Usually, this occurs in younger people and it's thought to be related to nutrient deficiencies causing food cravings, being more sedentary because of palpitations, and poor food choices but there isn't one specific reason and these reasons aren't confirmed.
Some people do lose weight after they start ATDs because their appetite decreases. Graves' disease does cause appetite to increase. If you can try cutting back on food portions, add essential oils like flaxseed or fish oils to your diet, and increase your exercise you're more likely to see results. Be sure to keep copies of your lab results too so you can make sure you're not overmedicated. After the first 6-8 weeks, most people are on doses of carb from 2.5-10 mg daily. TSH stays low for a long time and your dose should only be based on your FT4 level. You should be on the lowest dose needed to keep FT4 at least at mid-range.
Be sure you're getting adequate, but not excess protein. Adding protein to every meal and cutting back on carbs can also help. Congratulations on your upcoming wedding. Best, Elaine