What it is used for

Underactive thyroid (hypothyroidism)
After thyroid surgery or radioactive iodine treatment
Some types of thyroid cancer, to stop the thyroid being stimulated

Read our guides:

Hypothyroidism

Off-label uses

Doctors sometimes prescribe a medicine for a condition it is not officially licensed for. This is called off-label use. It is legal and common when there is good evidence it helps, and your doctor should explain why they are suggesting it.

Levothyroxine is not commonly used off-label.

How it works

Levothyroxine is a man-made version of thyroxine (T4), a hormone that controls your metabolism. It replaces the hormone your thyroid cannot make, restoring normal energy and body functions.

Side effects

Like all medicines, Levothyroxine can cause side effects, although not everyone gets them. Many are mild and ease after the first few weeks.

Common side effects

usually none once you are on the right dose
if the dose is too high, a fast heartbeat, sweating, shaking, feeling anxious, trouble sleeping or diarrhoea

Tell your doctor or pharmacist if these bother you or do not go away.

Serious side effects

These are rare. Call 111 or contact your doctor straight away if you get:

chest pain or a very fast or irregular heartbeat
a severe headache

Call 999 if you have signs of a serious allergic reaction, such as swelling of your lips, mouth, throat or tongue, or trouble breathing.

You can report any side effect to the Yellow Card scheme, run by the medicines regulator.

Stopping and withdrawal

Important safety information

Take it on an empty stomach, usually 30 to 60 minutes before breakfast or caffeine.
Leave 4 hours between it and iron, calcium or indigestion remedies.
You need blood tests to check your dose, especially when starting or if you become pregnant.

Questions about your medicine?

A pharmacist can answer questions about your medicines for free, without an appointment. You can also ask your GP for a medication review. Check if you can get free prescriptions.