Also Known As: Synthroid, Levothroid, Levoxyl, L-thyroxine, Levothyroxine

Levothyroxine, also L-thyroxine, synthetic T4, or 3,5,3',5'-tetraiodo-L-thyronine, is a synthetic form of thyroxine (thyroid hormone), used as a hormone replacement for patients with thyroid 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.

Levothyroxine is typically used to treat hypothyroidism.[4] It may also be used to treat goiter via its ability to lower thyroid-stimulating hormone (TSH), a hormone that is considered goiter-inducing.[5][6]

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