Levothyroxine, is a synthetic form of thyroxine (thyroid hormone), used as a hormone replacement for patients withthyroid problems. The natural hormone is chemically in the chiral L-form, as is the pharmaceutical agent. Dextrothyroxine (D-thyroxine) briefly saw research as an anticholesterol agent but was pulled due to cardiac side-effects.Synthroid is a prescription medication that can help treat hypothyroidism. The active ingredient in Synthroid is called levothyroxine sodium. Synthroid can help restore thyroid hormone balance. It is a man-made thyroid hormone identical to thyroxine, the hormone that's naturally made by the thyroid gland.