Levothyroxine, also known as L -thyroxine, synthetic T 4 or 3,5,3',5'-tetraiodo-L-thyronine, is a synthetic form of thyroxine (thyroid hormone). The natural hormone is chemically in the 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.

The EU has recently standardized on the use of the International Nonproprietary Name "levothyroxine" for the drug. Common brand names include "Thyrax", "Euthyrox", "Levaxin" and "Eltroxin" in Europe, and "Levoxyl" and "Synthroid" in the United States. There are also numerous generic versions approved by the Food and Drug Administration.