What is levothyroxine?

If you are diagnosed with hypothyroidism (underactive thyroid) your doctor will prescribe levothyroxine, which is a synthetic version of the thyroxine produced by the thyroid gland. Levothyroxine is very pure, and is unlikely to have side-effects when taken in the correct dose.

Content overview

What does should I have?

How should I take levothyroxine?

Will I need to change my dose?

What dose should I have?

Levothyroxine doses are dependent upon the person’s body weight and their blood test results. Most patients require between 100 and 150mcg a day, but the dose can be lower than 50mcg or up to 300mcg a day, depending on your needs. If you have severe hypothyroidism or are at risk of heart problems you can expect your doctor to start treating you with a low dose and increase the dose gradually. Patience is needed as it can take several months before you feel better and for the thyroid function tests to return to normal. During this period you will have regular thyroid function tests, usually every three months, and your dose may need to be adjusted according to the results of the tests.

How should I take levothyroxine?

Levothyroxine is best taken in the morning, with water, on an empty stomach, at least half an hour before eating and drinking anything. It is also best taken at least four hours apart from calcium, iron, cholesterol-lowering drugs (cholestyramine, colestipol), and multivitamin tablets, as these can also decrease absorption. Grapefruit also interferes with the absorption of levothyroxine. There are several other drugs that interact with levothyroxine. Always check with your doctor or pharmacy if you are on any other prescription or over-the-counter medication.

It is easy to miss a levothyroxine tablet, but because your body has a big reservoir of thyroxine, you will not notice a difference. However, it is important to take the tablets consistently every day as this can affect your blood test results and your health. Try to work out a system to help you take them every day, for example by setting up a regular reminder on your phone or by using a pill box.

Levothyroxine loses its potency over time and should be stored at room temperature away from moisture, heat and direct sunlight.

Will I need to change my dose?

Once the correct dose has been established it is unlikely to vary, although it is still important to have a blood test each year just to make sure. Too much levothyroxine will cause symptoms of an overactive thyroid and too little levothyroxine will not completely resolve symptoms of an underactive thyroid.

Around the time of menopause, and in postmenopausal women who are not prescribed oral hormone replacement therapy (HRT), some women may require their levothyroxine dose to be reduced. This is due to declining levels of oestrogen at this time.

Read our 'Your Guide to Hypothyroidism'

Read about liothyronine to treat hypothyroidism 

Please help us help others

With your support we can help people to live better with thyroid disease. Your donations also fund vital research to improve treatments.

Please consider making a donation or becoming a member

Donate to BTF

Become a BTF member