An underactive thyroid (hypothyroidism) is usually treated by taking daily hormone replacement tablets called levothyroxine.

Levothyroxine replaces the thyroxine hormone, which your thyroid does not make enough of.

You'll initially have regular blood tests until the correct dose of levothyroxine is reached. This can take a little while to get right.

You may start on a low dose of levothyroxine, which may be increased gradually, depending on how your body responds. Some people start to feel better soon after beginning treatment, while others do not notice an improvement in their symptoms for several months.

Once you're taking the correct dose, you'll usually have a blood test once a year to monitor your hormone levels.

If blood tests suggest you may have an underactive thyroid, but you do not have any symptoms or they're very mild, you may not need any treatment. In these cases, the GP will usually monitor your hormone levels every few months and prescribe levothyroxine if you develop symptoms.

If you're prescribed levothyroxine, you should take it at the same time every day. It's usually recommended that you take your tablet (or tablets) in the morning, although some people prefer to take them at night.

The effectiveness of the tablets can be altered by other medicines, supplements or foods, so they should be swallowed with water on an empty stomach, and you should avoid eating for 30 minutes afterwards.

If you forget to take a dose, take it as soon as you remember, if this is within a few hours of your usual time. If you do not remember until later than this, skip the dose and take the next dose at the usual time, unless advised otherwise by a doctor.

An underactive thyroid is a lifelong condition, so you'll usually need to take levothyroxine for the rest of your life.

If you're prescribed levothyroxine because you have an underactive thyroid, you're entitled to a medical exemption certificate. This means you do not have to pay for your prescriptions. See getting help with prescription costs for more information on this.

Side effects

Levothyroxine does not usually have any side effects, because the tablets simply replace a missing hormone.

Side effects usually only occur if you're taking too much levothyroxine. This can cause problems including sweating, chest pain, headaches, diarrhoea and being sick.

Tell the doctor if you develop new symptoms while taking levothyroxine. You should also let them know if your symptoms get worse or do not improve.

In the UK, combination therapy – using levothyroxine and triiodothyronine (T3) together – is not routinely used because there's insufficient evidence to show it's better than using levothyroxine alone (monotherapy).

In most cases, suppressing thyroid-stimulating hormone (TSH) using high-dose thyroid replacement therapy should be avoided because it carries a risk of causing adverse side effects, such as atrial fibrillation (an irregular and abnormally fast heart rate), strokes, osteoporosis and fracture.

However, this type of treatment may sometimes be recommended in cases where a person has a history of thyroid cancer and there's a significant risk of it reoccurring.

It's important for the health of you and your baby that an underactive thyroid is treated properly before you become pregnant.

Tell a GP if you're pregnant or trying to become pregnant and you have hypothyroidism. They may refer you to a specialist for treatment and monitoring during your pregnancy.