“If my daughter has an overactive thyroid, will she be overweight?”

Overactive thyroid usually increases metabolism rate so there is usually unexplained weight loss not weight gain. If she takes anti-thyroid medication, the common starting treatment for hyperthyroidism and the dose is too high or for too long a time period, then the thyroid hyperactive changes to underactivity (hypothyroidism) and that can induce weight gain. Key is to have close endocrinology followup with periodic exam and lab testing to track thyroid hormone blood levels to guide treatment decisions.

The thyroid gland is supposed to make normal amount of thyroid hormone to help us maintain energy balance. But sometimes it may produce too little or too much. When it makes to little we call it under-active. When it makes too much thyroid hormone, it causes thyrotoxicosis, which is thyroid over activity. When it makes too much, it causes metabolic over-activity, which causes weight loss not weight gain. The opposite is true when it is under-active.
Please don't confuse it with high TSH. In hyper thyroid (toxic thyroid) TSH is supposed (low). The opposite is true with under-active thyroid, with some exceptions.
So, in hyperthyroidism, weight loss, not weight gain is expected.
Good luck.

Overactive thyroid is more likely to result in weight loss, not gain, over time.

No, the most common symptoms of hyperthyroidism (overactive thyroid) are tremors, shaking, palpitations, frequent diarrhea, and weight loss.