If you have an underactive thyroid, your healthcare provider may prescribe liothyronine as part of thyroid replacement therapy. The drug is also approved for treating various types of goiters and can be used in certain thyroid diagnostic tests. Liothyronine comes in tablet form, is available in three different strengths, and is generally taken once daily. Potential side effects include sweating, nervousness, and headache.

What Is Liothyronine?

Liothyronine sodium (Cytomel®) is a prescription medication that is a manufactured version of a certain thyroid hormone. Even though it is synthetic, this drug is identical to the naturally occurring hormone liothyronine (also known as T3 or triiodothyronine). It is approved to treat an underactive thyroid (known medically as hypothyroidism) and goiters. Liothyronine is also used in certain thyroid diagnostic tests.
(Click What Is Liothyronine Used For? for more information, including possible off-label uses.)

How Does It Work?

The thyroid gland makes two different thyroid hormones: levothyroxine (T4) and triiodothyronine (T3). Although T3 is much more active than T4, the thyroid usually produces more T4 than T3. The body can convert the T4 hormone into T3 as necessary. If your thyroid does not make enough hormones, there are a few different options to increase your levels.
Some forms of thyroid replacement combine T4 and T3 (such as natural thyroid replacement made from pig thyroids). However, such products are usually not the preferred option for most people. Synthetic (manufactured) thyroid hormones like liothyronine are less likely to cause allergic reactions, since they are not made from animal thyroids, and may provide for more precise control of thyroid levels.
Although most people take a synthetic T4 thyroid replacement product (such as Synthroid®), some people may benefit from T3 replacement (such as liothyronine), either alone or in combination with a T4 medication. Some people may not convert T4 into T3 appropriately, in which case a T3 product like liothyronine would work better.
Last reviewed by: Kristi Monson, PharmD
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