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From SNPedia

Levothyroxine (INN, USAN) or L-thyroxine is a synthetic thyroid hormone that is chemically identical to thyroxine (T4), which is naturally secreted by the follicular cells of the thyroid gland. It is used to treat thyroid hormone deficiency, and occasionally to prevent the recurrence of thyroid cancer. (Wikipedia, edited 2017)

A common brand name of this drug is Synthroid. It is sometimes referred to as L-T4 to distinguish the medicine from the hormone T4.

Side effects from excessive doses include weight loss, trouble tolerating heat, sweating, anxiety, trouble sleeping, tremor, and fast heart rate. Use is not recommended in people who have had a recent heart attack. Use during pregnancy has been found to be safe. It is recommended that dosing be based on regular measurements of TSH and T4 levels in the blood. Much of the effect of levothyroxine is following its conversion to triiodothyronine (T3). (Wikipedia, edited 2017)

L-T4 requires the gene products of DIO1 and DIO2, the iodothyronine deiodinase genes, to converted it into the active thyroid hormone T3 within the body. Another deiodinase gene, DIO3, converts excess T4 into Reverse T3 and clears excess T3.

See related SNPedia pages on medical conditions it is often used to treat: Hypothyroidism, Autoimmune thyroiditis, Hashimoto's thyroiditis, Thyroid cancer.

Another medicine, Liothyronine, the synthetic form of the active hormone T3, is sometimes considered for treatment of thyroid hormone deficiency, usually in combination with levothyroxine.