Thyroxine levothyroxine: "Excessive dosage or a too rapid increase in dosage. A weight-loss drug and should not be used as such. Levothyroxine is a naturally occurring hormone produced by the thyroid glands also known as goiter. Levothyroxine sodium. Synthroid is the most commonly prescribed of all the thyroid supplements. Common side effects produced by Synthroid include fatigue, increased appetite, weight loss, heat intolerance, and fever. Side effects for this reason the correct dose of Synthroid is difficult to find. Synthroid is to be used only by the patient for whom it is prescribed."