Can you use levothyroxine for weight loss?

Yes. You can boost metabolism but will cause fast heartbeat or irregular heartbeat so need supervision by a doctor(need to be prescribed by a doctor).
Only if hypothyroid. If a person has hypothyroidism (low thyroid hormone levels), she can be treated with levothyroxine, which would be her replacement thyroid hormone. If her low thyroid condition caused her to gain weight (which does occur in hypothyroidism), she will likely lose the weight when her hormone levels normalize by taking levothyroxine.