Levothyroxine, also known as L-thyroxine, synthetic T4 or simply 3,5,3',5'-tetraiodo-L-thyronine, is a synthetic form of thyroxine (thyroid hormone). As the hormone is chemically in the L-form (instead of D, see chirality), it is metabolised much slower than physiological thyroid hormone, which has a shorter half-life. The EU have recently standardised the use of the name "levothyroxine" for the drug.