Endocrine System Home > Synthroid

Synthroid is typically prescribed to treat an underactive thyroid and a variety of thyroid problems. This manufactured drug is identical to the naturally occurring hormone levothyroxine (T4). As with any medication, side effects are possible with Synthroid, such as insomnia, hair loss, and a rapid heart rate. The medication comes in tablet form and is taken once a day in the morning.

What Is Synthroid?

Synthroid® (levothyroxine sodium) is a prescription medication that is a manufactured version of a certain thyroid hormone. Even though it is synthetic, the drug is identical to the naturally occurring hormone levothyroxine (also known as T4). It is used to treat an underactive thyroid (known medically as hypothyroidism) and to treat a variety of thyroid problems (including certain types of thyroid cancer).
(Click Synthroid Uses for more information, including possible off-label uses.)

Who Makes Synthroid?

Brand-name Synthroid is made by Abbott Laboratories.

How Does It Work?

The thyroid gland makes two different thyroid hormones: levothyroxine (T4) and triiodothyronine (T3). Typically, the thyroid produces much more T4 than T3 (however, T3 is much more active than T4). The body can convert the T4 hormone into T3 as necessary. If your thyroid does not make enough thyroid hormones, there are a few different ways to increase your levels.
Some forms of thyroid replacement combine T4 and T3 (such as natural thyroid replacement made from pig thyroids). However, because the body converts T4 into T3 as needed, most people can successfully take just T4 (such as with Synthroid). Alternatively, just T3 can be taken (as products such as Cytomel®). Currently, most people take just T4 (such as Synthroid).
Written by/reviewed by:
Last reviewed by: Kristi Monson, PharmD;
Last updated/reviewed:
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