Thyroid hormone replacement
Levothyroxine
The standard treatment for an underactive thyroid — a daily tablet that simply replaces the thyroid hormone your body is missing, taken on an empty stomach and adjusted by a blood test.
What is Levothyroxine?
Levothyroxine replaces the hormone (thyroxine, T4) that an underactive thyroid does not make enough of. It is one of the most commonly prescribed medicines in the UK, usually taken once a day on an empty stomach, and the amount is guided by a blood test (TSH). It is normally lifelong, works gradually over weeks, and needs separating from certain other tablets and foods that block its absorption.
Education and reference only. This is a plain-language guide to Levothyroxine — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.
What it is
Levothyroxine is the standard treatment for hypothyroidism — an underactive thyroid gland that does not make enough thyroid hormone. It is a manufactured version of the body's own main thyroid hormone (thyroxine, or T4), so treatment simply replaces what is missing and restores normal levels. It is one of the most frequently prescribed medicines in the UK, is usually taken for life, and once the right amount is settled most people feel completely well on it. It is taken as a daily tablet (with a liquid available).
How it works
Levothyroxine is an identical copy of thyroxine (T4), the main hormone the thyroid gland normally releases. The body converts it into the active hormone (T3), which sets the pace of metabolism in nearly every tissue — energy, temperature, heart rate, digestion and mood. By replacing the missing hormone, levothyroxine returns these processes to normal. Because it works at the level of the body's hormone balance, its effect builds up over several weeks, and the correct amount is judged by measuring the pituitary hormone TSH, which falls back into the normal range when replacement is right.
What it treats
Conditions Levothyroxine is used for
Practical use
How to take Levothyroxine
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- Take it once a day on an empty stomach — usually first thing in the morning, at least 30–60 minutes before food, tea or coffee (or at bedtime, well after eating, if that suits you better).
- Separate it by several hours from calcium and iron supplements, indigestion (antacid) remedies and some other tablets, which block its absorption.
- Keep taking it consistently and long-term — it works gradually, and the benefit only continues while you take it.
- Have your TSH blood test as advised (usually every several weeks after a change, then yearly once stable) so the amount can be fine-tuned.
- If you miss a dose, take it as soon as you remember that day; if you forget until the next day, skip it and carry on — do not double up.
Weighing it up
Advantages & disadvantages of Levothyroxine
Advantages
- Simply replaces a missing hormone, so once the amount is right most people feel entirely normal.
- Once-daily, inexpensive, and effective, with a clear blood test (TSH) to guide it.
- Usually very well tolerated when the level is correct.
Disadvantages
- Needs to be taken on an empty stomach and separated from certain foods and tablets.
- Works slowly, so getting the amount right takes several rounds of blood tests.
- Too much can strain the heart and bones over time, so the level is monitored.
Practical use
Good to know
For levothyroxine to be absorbed properly, timing matters: it is best taken on an empty stomach — typically first thing in the morning, at least 30 minutes to an hour before food, tea or coffee — and kept away from certain tablets that block its absorption, especially calcium, iron and indigestion remedies, which should be taken several hours apart. It works slowly, so the dose is adjusted in steps every several weeks guided by a TSH blood test rather than by how you feel day to day. Too little leaves you tired, cold and low; too much can cause palpitations, anxiety, tremor and, over time, affect the heart and bones. Some people are kept on the same brand if they are sensitive to changes.
Who should not take it / use with caution
- People with an overactive thyroid (levothyroxine is a replacement, not a treatment for too much thyroid hormone).
- People with certain untreated heart conditions or an untreated adrenal hormone deficiency, where it is started with particular care.
- In older people and those with heart disease it is usually started low and increased slowly to avoid straining the heart.
Monitoring
- Thyroid blood test (TSH, with T4 when needed) to set and check the amount
- Symptoms of over- or under-replacement
- Heart rate in older people and those with heart disease
Side effects
- When the amount is correct, levothyroxine usually causes no side effects — it is simply replacing your own hormone.
- Signs of too much (over-replacement): palpitations, a fast heartbeat, tremor, anxiety, feeling too hot, weight loss and difficulty sleeping — the dose is then reduced.
- Signs of too little (under-replacement): persistent tiredness, feeling cold, weight gain, constipation and low mood — the dose is then increased.
Key interactions
- Calcium, iron, and indigestion (antacid) remedies reduce its absorption — take them several hours apart.
- Some medicines (including certain seizure medicines, rifampicin, and oestrogens) can change how much levothyroxine you need.
- Its effect on other medicines can also change — for example warfarin may need review when thyroid levels change.
Available as: Tablets in several strengths; a liquid is available for people who cannot swallow tablets or need fine adjustment.
Answers
Levothyroxine: frequently asked questions
Why do I have to take levothyroxine on an empty stomach?
Food, tea, coffee and especially calcium and iron supplements reduce how much levothyroxine is absorbed. Taking it on an empty stomach — usually first thing in the morning, at least 30–60 minutes before eating or drinking anything other than water — gives steady, reliable absorption. Taking it at bedtime, well after your last food, is an alternative that works for some people.
How long until I feel better?
Levothyroxine works gradually. It can take a few weeks to feel the benefit, and the amount is adjusted in steps guided by a TSH blood test roughly every 6–8 weeks until it settles. Once stable, most people feel completely well and have a yearly check.
Do I have to take it for life?
Usually yes. In most people the thyroid does not recover, so replacement is lifelong — but it is simply topping up a hormone your body needs, and once the level is right it should not cause problems. Do not stop it without advice, as your symptoms will return.
Can I take my other tablets at the same time?
Keep levothyroxine well apart (several hours) from calcium and iron supplements and indigestion remedies, which block its absorption. Most other medicines are fine, but check with your pharmacist — and if your thyroid dose or another medicine changes, your levels may need rechecking.
Authoritative sources
- BNF: Levothyroxine sodium.
- electronic Medicines Compendium (SmPC): Levothyroxine.
- NICE NG145: Thyroid disease – assessment and management.
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