Endocrine
Levothyroxine
Thyroid hormone replacement — Replaces thyroid hormone in an underactive thyroid — titrated to blood tests.
Education and reference only. This is a plain-language class overview — it deliberately contains no doses. Always check the current Summary of Product Characteristics (SmPC), the BNF and your local formulary before prescribing or administering any medicine.
What it is
Levothyroxine is the standard treatment for hypothyroidism (an underactive thyroid), replacing the hormone the gland no longer makes in sufficient amounts. It is one of the most commonly prescribed medicines.
How it works
It is a synthetic version of thyroxine (T4), which the body converts to the active hormone T3. Restoring normal thyroid hormone levels returns metabolism, energy and many body systems to normal.
In practice
In practice levothyroxine is titrated to the TSH, which is checked several weeks after any change because the level takes time to settle. Take it on an empty stomach, separated from calcium, iron and some other drugs that block absorption. Older patients and those with heart disease are started cautiously, and pregnancy usually increases requirements — so it needs prompt review.
Examples
Practical use
How to take it & use it well
- Take it on an empty stomach with water, ideally first thing in the morning, then wait before eating or drinking anything else.
- Keep to the same routine and time every day so your levels stay steady.
- Separate it well from calcium supplements, iron, indigestion remedies and your morning coffee, as these can stop it being absorbed properly.
- If you forget a dose, take it as soon as you remember unless it is nearly time for the next one; never double up to catch up.
- Do not stop or change how you take it without advice, even once you feel well, as treatment is usually lifelong.
- Store the tablets in a cool, dry place away from direct heat and moisture.
Common uses
- Primary hypothyroidism
- After thyroid surgery or radioiodine
- Congenital hypothyroidism
Monitoring
- TSH (and sometimes T4) a few weeks after starting or changing dose, then periodically
- Symptoms of over- or under-treatment
- Earlier review in pregnancy
Weighing it up
Advantages & disadvantages
Advantages
- It replaces the thyroid hormone your body cannot make enough of, relieving tiredness, weight gain, low mood and feeling cold.
- When the dose is right it usually controls symptoms fully and lets you live a normal life.
- It is a well-established, inexpensive medicine with decades of safe use behind it.
- It is taken just once a day, which makes it easy to fit into a routine.
- It is safe to continue in pregnancy under monitoring, and is often important for a healthy pregnancy.
Disadvantages
- Finding the right dose takes time and needs blood tests, so symptoms may not settle straight away.
- Too much can cause a fast heartbeat, shakiness, sweating and trouble sleeping, while too little leaves you under-treated.
- Absorption is easily upset by food, coffee and certain supplements, so the timing rules matter.
- Your needs can change over time, in pregnancy or with other illnesses, so regular monitoring is needed.
- It is a replacement, not a cure, so it usually has to be taken for life.
Key safety principles
What to watch for
- Too much causes symptoms of an overactive thyroid and, long-term, atrial fibrillation and bone loss.
- Start low and go slow in older people and those with ischaemic heart disease.
- Absorption is reduced by calcium, iron and some other medicines — separate the doses.
Key interactions
What to avoid or check alongside
- Calcium and iron supplements bind to levothyroxine in the gut and reduce how much you absorb, so space them apart.
- Indigestion and heartburn remedies containing calcium, aluminium or magnesium can lower absorption if taken together.
- Coffee taken too close to your dose can reduce its effect.
- Some medicines for seizures and certain antibiotics can change how much thyroid hormone you need.
- If you take a blood thinner, your thyroid dose can affect how that works, so tell your team about any changes.
- Soya products and high-fibre supplements can interfere with absorption when taken at the same time.
Patient & carer advice
- Take on an empty stomach, ideally 30–60 minutes before breakfast
- Keep to the same brand/formulation where possible
- Do not double up if you miss a dose; ask if unsure
Answers
Levothyroxine: frequently asked questions
Why do I have to take it on an empty stomach?
Food, milk and some drinks reduce how much levothyroxine your body absorbs. Taking it with water on an empty stomach and waiting before breakfast gives the most reliable effect.
How long until I feel better?
It often takes several weeks to notice a clear difference, and your dose may need adjusting after blood tests before symptoms fully settle.
Can I take it at night instead?
Some people take it at bedtime, well after their last food. The key point is taking it consistently and away from food, so discuss any change with your prescriber.
Do I need regular blood tests?
Yes. Blood tests check your thyroid levels and guide any dose changes, especially when you first start, after a change, or in pregnancy.
What if I miss a dose?
Take it when you remember unless your next dose is almost due, then skip the missed one. Do not take two doses together to make up for it.
Authoritative sources
Always verify against the source
This overview is for orientation. For doses, interactions, contra-indications and the full monograph, use:
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