Levothyroxine (T4)

Your body makes two main hormones, thyroxine (T4) and tri-iodothyronine (T3). Levothyroxine is a man-made version of thyroxine and is given to patients to replace the thyroxine that their body does not produce.

Although it starts working straight away, it takes around 8-10 days to get properly into your system and for you to start to feel better.  Having said that some people have reported a reduction in their symptoms before that.

Treatment is usually started at either 25mcg or 50mcg per day, depending on the severity of the condition.  Testing is carried out at various intervals and dosages increased over the following weeks and months until the test results show within normal range and your symptoms are relieved.

The tests that are usually done to check your levels are the thyroid stimulating hormone (TSH) test and the free thyroxine (FT4) test.

Generally, people feel well when their levels are within range but everyone is different and you may feel best at a particular level within that range.

According to the BMA’s booklet, “Understanding Thyroid Disorders“, by Dr Athony Toft, many people do not feel well unless their levels are at the bottom of the TSH range or below and at the top of the FT4 range or a little above. Dr Toft also stated in a 2010 issue of Pulse Magazine:

“The appropriate dose of levothyroxine is that which restores euthyroidism and serum TSH to the lower part of the reference range – 0.2-0.5mU/l.

In this case, free thyroxine is likely to be in the upper part of its reference range or even slightly elevated – 18-22pmol/l. Most patients will feel well in that circumstance.

But some need a higher dose of levothyroxine to suppress serum TSH and then the serum-free T4 concentration will be elevated at around 24-28pmol/l.

This ‘exogenous subclinical hyperthyroidism’ is not dangerous as long as serum T3 is unequivocally normal – that is, serum total around T3 1.7nmol/l (reference range 1.0-2.2nmol/l).”

There are various brands and generics (copies of brands) of levothyroxine manufactured by different pharmaceutical companies.  These can vary in their ingredients. Some contain lactose and therefore for people who are lactose intolerant it’s best to find a lactose free levothyroxine.

We are finding that more and more people are intolerant of another ingredient, mannitol, which has been used to replace lactose in some tablets.

Some people are intolerant of some of the fillers and binders within the levothyroxine tablet and we know of some people who are intolerant of a particular type of levothyroxine which is called anhydrous levothyroxine.

For most, there will not be any problem when taking levothyroxine.  For those people that do have problems with some of the ingredients, try to find out which ingredient or which generic you are having issues with.

There are brands of tablets that are lactose free and mannitol free and there are compounding pharmacies (pharmacies that make the tablet to order) that can make levothyroxine tablets with very little or no fillers and binders or with fillers and binders that are less likely to cause problems.

Thyroid UK has a list of all common thyroid medications including the ingredients which you can find here.

Is your brand constantly being switched?

We are finding that people are being given various generic brands, depending on which generic the pharmacy has in stock at that time (often due to cheaper costs for them).

If this happens to you and you want to only have one particular brand, do visit your GP and ask them to organise for that particular generic to be placed on every prescription.

In May 2021, the MHRA published in their “Latest Advice for Medicines Users” (Volume 14, Issue 10 May 2021) a report that informed prescribers that, “If a patient reports persistent symptoms when switching between different levothyroxine tablet formulations, consider consistently prescribing a specific product known to be well tolerated by the patient.”

Do inform your prescriber of this if you want to be prescribed only one particular generic.

To download and read this advice go to: https://assets.publishing.service.gov.uk/media/60a50b7ae90e073573036876/May-2021-DSU-PDF.pdf

To download our list of medications for hypothyroidism click here.

How should I take levothyroxine?

Levothyroxine should be taken on an empty stomach with a glass of water. Do not eat for at least half an hour.  Food in your stomach stops absorption by up to 40%.

Most people take their levothyroxine in the morning, on rising but some people find it better to take it before bed especially if they are on medications or supplements that can interfere with absorption.

Food and drugs that can cause problems

There are various foods and drugs that can either affect absorption or cause other effects.  Read more about food and drug interactions.

Smoking

Smoking can mask hypothyroidism.  It causes your thyroxine levels to rise and if you give up smoking, studies show that thyroxine levels reduce possibly needing short term levothyroxine treatment. Smoking is also related to autoimmune thyroid disease so giving up may be prudent if the disease is in your family.

For more support check out our online community:
http://www.healthunlocked.com/thyroiduk

Glossary

Autoimmune – patients with autoimmune diseases frequently have antibodies circulating in their blood that target their own body tissues

Hypothyroidism – a term used to describe an under-active thyroid gland

Levothyroxine – a synthetic thyroid hormone commonly given to treat an under-active thyroid. It is also known as L-thyroxine

Thyroxine – the main hormone secreted into the bloodstream by the thyroid gland. It is the inactive form

Date updated: 29.7.25 (V1.6)
Review date: 17.7.28