Levothyroxine is a thyroid hormone. It is needed for all kinds of processes in the body. Such as body growth, metabolism and mental ability.
When you have a slow-acting thyroid gland. In this you are sluggish, cold, lethargic and your weight increases. Also together with medicines that inhibit the thyroid gland: these can cause a shortage of thyroid hormone. Levothyroxine remedies this.
It takes several days before you notice anything. Usually you have to take the medicine for life.