What Causes Weight Gain Despite Regular Exercise And Thyroid Medication?

my 27 year old daughter has hypothyroidism. She takes her meds as prescribed. She eats 1200 cals a day, very strict with it. She exercises 6 days a week, for about 40 mins a day. She has trained for and run 5 ks even. Still she is gaining weight. She lost about 40 lbs when she started her thyroid meds, but has gained back 20 or so even though she continues her efforts. Could she have cushings? Dr was skeptical and said that is just the way it is sometimes. I feel something must be wrong. She is around 50lbs over weight now.