This information is for people who have an underactive thyroid. It tells you about liothyronine plus levothyroxine, a treatment used for an underactive thyroid. It is based on the best and most up-to-date research.
Do they work?
Research shows that taking liothyronine together with levothyroxine doesn't help you more than taking just levothyroxine.
What are they?
Liothyronine is a man-made version of the hormone triiodothyronine (T3), which is made by the thyroid gland. Doctors sometimes use it in emergencies to treat severe symptoms of an underactive thyroid.  It starts to work more quickly than levothyroxine. It comes as tablets and injections.
Levothyroxine is a man-made version of the hormone thyroxine (T4). Thyroxine is the hormone that your body doesn't make enough of when you have an underactive thyroid. Levothyroxine is the treatment doctors usually use alone for an underactive thyroid.
How can they help?
Research shows that levothyroxine alone works just as well as levothyroxine plus liothyronine. Most people don't get any extra benefit from taking liothyronine as well.  
How do they work?
Levothyroxine and liothyronine work by replacing your body's own thyroid hormones T4 and T3. Without enough of these hormones, you become sluggish and tired. This is because your body doesn't get the energy it needs. Your metabolism slows down. This can cause many problems. For example, your bowel muscles may slow down, so you get constipated. Or your heart may slow down or your cholesterol level may rise, putting you at risk of heart problems.
Taking synthetic versions of these hormones gives back to your body the T4 and T3 it needs to keep your metabolism at the correct level. Researchers hoped that using both these hormones together would help more than just one.
Can they be harmful?
We didn't find any research on the harmful effects of liothyronine alone. But in studies, taking the two drugs together caused about the same amount of side effects as taking just levothyroxine. 
In older people, taking too much levothyroxine has been linked with getting a fast and irregular heart beat (atrial fibrillation). About 1 in 5 people over 60 who have low levels of TSH get this problem. But it's not clear whether the problem is because of taking levothyroxine, or because of having an underactive thyroid. 
If you're a woman and have already been through the menopause, taking too much levothyroxine can cause thinning of the bones (osteoporosis). This can happen to women who've been taking high doses of levothyroxine for 10 years. 
If your bones get thinner, you have a higher risk of getting fractures. But there's no evidence that women who take levothyroxine are more likely to have a bone fracture than women who don't. 
If you have an underactive thyroid without having any symptoms, there's a small chance that taking levothyroxine will make you feel more anxious.  There's also a small risk of heart problems, such as chest pain (angina) and a fast, irregular heart beat ( atrial fibrillation). In one study of 18 people, two stopped taking levothyroxine because of heart problems.