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Thyroid medication (Levothyroxine): Replacement, dosage and side effects

Thyroid pills like levothyroxine are medications that imitate the body's own thyroid hormones. They work in the same way as the natural hormone messenger and are used when the thyroid gland produces too little thyroid hormones itself (hypothyroidism).

Please find more information on thyroid medication in general, its uses, dosage and side effects in the following text.

What is the difference between thyroid hormones and thyroxine?

Thyroid hormones are messenger substances produced by the body itself. Levothyroxine (or short L-thyroxine) is the manufactured form of the thyroid hormone thyroxine. It is used for hormone replacement therapy if your body produces too little thyroxine on its own. Biological thyroxine and manufactured levothyroxine work the same.

Weak thyroid gland

The thyroid produces two types of thyroid hormones. They are called thyroxine (T4) and triiodothyronine (T3). However, under certain circumstances, the thyroid gland may produce only little thyroid hormones.

Iodine deficiency, for example, is a common reason for hypothyroidism in southern Germany. It is a basic part of T4 and T3 and with little or no iodine, the thyroid hormone production decreases as well.

Another typical reason for hypothyroidism is an inflammation of the thyroid gland, it is called Hashimoto’s thyroiditis. After thyroid surgery or radiotherapy of the thyroid for other medical reasons, hypothyroidism may follow due to the fact, that there is often not enough thyroid tissue left to produce the hormones.

Substitution with levothyroxine

Hypothyroidism it treated with artificially produced thyroid hormone tablets like L-thyroxine, which corresponds to the natural T4. The thyroid gland itself mainly produces T4. The body builds up a hormone supply and can be converted into the so-called T3, the more active form of thyroid hormones (triiodothyronine). T3 is primarily responsible for the various tasks of the thyroid gland.

Structure and function of the substance do not differ from the body's own thyroxine (T4), so it is "bioidentical". This means that the body cannot distinguish between endogenous and synthetically produced thyroid hormones and is not affected in any way in its mode of operation.


How does thyroid hormone replacement work?

Thyroxine / levothyroxine stimulates the energy metabolism. It is the precursor for another thyroid hormone: triiodothyronine (T3). Both thyroid hormones are required for the regulation of numerous processes in the body, for energy metabolism. The stimulate it.

And they are also involved in protein, carbohydrate, lipid, nucleic acid and vitamin metabolism. In addition to basic effects on metabolism, they influence processes in the stomach and intestines, heart and circulation as well as the nervous system and psyche. They also control the growth of skin, hair and nails.

Treatment of hypothyroidism

Hypothyroidism is a very common medical problem and requires lifelong substitution of thyroid hormones. Usually, it takes up to three weeks after starting thyroxine, until the full treatment effect is achieved.

Treatment of goiter

Other areas of application for thyroxine include:

  • the treatment of a benign enlargement of the thyroid gland (known as goiter)
  • autoimmune Hashimoto thyroiditis (inflammation of the thyroid gland)
  • and special cases of hyperthyroidism

A goiter, an enlarged thyroid gland, is usually caused by a lack of iodine, which itself is needed in order to produce thyroid hormones. A lack of dietary iodine leads to a decreased thyroid hormone production. The thyroid gland starts to grow in order to compensate for the lack of hormones. This may help a little, but not enough.

Other application fields

  • Thyroxine may also be used:
  • After operations of the thyroid gland
  • In malignant tumors

As part of a so-called suppression test, in order to examine the thyroid function

Also, occasionally, thyroxine is misused by some people with normal thyroid function to lose weight. With respect to possible side effects, we strongly discourage this. Even deaths due to overdoses are known with this medicine.

Billions of daily doses prescribed in Germany

According to data of one of the big German health insurances, thyroxine was the second most prescribed drug in Germany in 2012 with 17 million prescriptions.


What do I have to consider when taking levothyroxine?

Thyroid hormones are used to treat hypothyroidism. Depending on the cause, they need to be taken temporarily or lifelong. It is important to follow some rules in order to optimally supply the body with the vital hormones.

Often, patients do not only take levothyroxine alone but combination pills with iodine, which the body needs to produce thyroid hormones.

Finding the right dose

The hormone preparations are available in different dosages. The daily dosage varies from person to person and depends on your so-called TSH-level. Thyroid hormones should always be taken in consultation with a doctor.

It is often not so easy to find the right dose for the individual. The doctor will slowly approach the amount that is best for you together with you. Using the thyroid value TSH, he will first determine the starting dose, then re-check the blood test regularly and adjust the dose as necessary. TSH-level and your personal feeling of well-being determine your thyroxine dose. Therefore, it is important that you give your doctor feedback on how you are doing.

Start low and slowly

It is important to start the therapy slowly so that your body has time to get used to the hormones supplied. The initial dose should be as small as possible and should only be increased slowly in small steps until the "normal" hormone concentration is reached. Especially in people with cardiovascular disease, physicians must be a little careful.

Best in the morning

L-thyroxine should always be taken at the same time, ideally in the morning, fasting, half an hour before breakfast, with a large glass of water.

Don´t switch your medicine

The effect of L-thyroxine depends on the composition of the preparation. This may vary from manufacturer to manufacturer. Even if all preparations contain the same active ingredient in the same quantity, the body may react differently to certain binders and additives. If you intend to change the preparation, you should therefore consult your doctor.

Possible interactions

Taking other medicines at the same time can cause various interactions. Glucocorticoids (cortisone), beta-blockers (lowering blood pressure) and birth control pills, for example, can reduce the effect of L-thyroxine. In addition, calcium and iron supplements and medicines used to treat gastric reflux (so-called antacids) can hinder the absorption of L-thyroxine from the intestine.

In addition, patients with diabetes who take metformin or insulin must be careful when taking levothyroxine as it may reduce the blood sugar lowering effect. In addition, L-thyroxine can increase the effect of anticoagulants like coumadin, for example.

Certain foods like milk and soy products can also reduce the absorption of thyroid hormones.

Also: Patients who are allergic to an active ingredient or suffer from acute heart muscle inflammation must not take L-thyroxine.

What about thyroid hormones and coffee?

Thyroxine should not be taken together with a cup of coffee. The caffeine can delay and impair the absorption of thyroid hormones into the bloodstream.

If the thyroxine preparations are taken at the same time as coffee, the absorption of the hormone from the intestine into the bloodstream can be delayed by around 40 minutes. And what is even more important: the so-called bioavailability of the hormone, the quantity of hormone reaching the body cells, also drops considerably - apparently by around 30%. This also applies in a similar way to other caffeine-containing drinks such as coke, energy drinks or green and black tea.

Soya, calcium and PPIs reduce absorption

Thyroid hormones should always be taken in the morning on an empty stomach with a large glass of tap water. You should also wait at least 30 minutes with breakfast - and coffee - after taking the medication. Even a sip of coffee right after taking the medicine can reduce its absorption.

In addition, other foods like soya products can also hinder the absorption of L-thyroxine. The same applies to some drugs, including calcium preparations and PPIs (proton-pump inhibitors like Pantoprazole & Co) which reduce stomach acid.

For coffee-addicts, who cannot survive without coffee in the morning, physicians may switch the time for taking your pills to the evening. However, taking L-thyroxine may lead to sleep disorders.

What do I do if I have forgotten to take my thyroxine?

If you have forgotten to take your thyroid hormones (L-thyroxine), you should continue taking the next tablet on the following day as usual. Do not switch or interrupt your rhythm.

Usually, forgetting thyroxine once does not have a negative effect on hormone levels, since the regular intake of the hormone has led to a constant and steady blood level. Also, do not "compensate" by taking a double dose the next day, if you forgot your medication.

A few important points to remember

  • Regular (daily) intake of L-thyroxine is important for a stable hormone level.
  • If possible, your medicine should always be taken at the same time, preferably in the morning, fasting on an empty stomach.
  • If you forget to take one or two tablets, you should not take the missed dose at a later point.
  • Taking thyroxine in the evening can cause sleep problems. It increases alertness and can cause sleep disorders as an undesirable side effect.

Tip: Alarm clocks and apps may help reminding you to take your medicine regularly without missing a dose.

Hypothyroidism: Who needs treatment?

Patients with a so-called manifest hypothyroidism, where thyroid hormone levels are low and patients suffer from symptoms, must always be treated. Patients with a so-called latent hypothyroidism (only elevated TSH and normal T3 and T4), on the other hand, do not always necessarily have to take levothyroxine as long as they are free of symptoms. Thus, in some cases, physicians may wait and recheck blood levels without prescribing thyroid hormones. Yet, age and underlying diseases also play a role when deciding what to do.

How long do I have to take levothyroxine?

How long you must take a thyroid hormone replacement depends on the underlying thyroid disease as well as on the degree of malfunction. Usually, in hypothyroidism and after thyroid surgery you must take levothyroxine long term and lifelong. If you suffer from goiter, sometimes a few months are enough.

Side Effects

What side effects can occur with levothyroxine?

Normally, thyroid hormones are well tolerated. Yet, incorrect dosages (too high or too little) can lead to side effects. As mentioned earlier, restlessness, heart racing and nervousness can be signs of a thyroxine overdose.

If, for example, the thyroxine dosage is set too high or if the dosage is increased too quickly at the start of the therapy, the following typical symptoms may occur:

  • Agitation
  • Heart racing (tachykardia)
  • Irregular heart beat (palpitations)
  • Other cardiac arrhythmias
  • Chest pain with tightness and angina pectoris (rather rare)
  • Nervousness and sleeplessness
  • Ravenous hunger, Diarrhoea
  • Tremor, heat sensation and excessive sweating
  • Menstrual disorders
  • Headaches
  • Muscle weakness and hair loss

Challenging: Finding the right dose

Without thyroid hormones, the body cannot perform basic functions. If the thyroid gland is underactive, it is therefore important to replace the missing hormone.

However, finding the right dose can sometimes be challenging due to a small therapeutic range. It describes the dose with which a drug works without causing side effects. For thyroxine, this range is rather small.

Step by step

Substitution of thyroid hormones must therefore be initiated carefully. The starting dose lies usually around 25-50 µg of L-thyroxine. The dose is increased gradually at longer intervals. Regular blood tests help to determine the proper dosage. Depending on the disease, the final dose often reaches from 75 and 200 µg of L-thyroxine.

In elderly and/or cardiac patients, physicians must be even more careful. These patients may react more sensitively to the hormones. Also, diabetics can have problems with their sugar levels since thyroid hormones can increase blood sugar.

Adapting your dose

Our hormone system follows a very complex, finely balanced cycle. In some situations, we need more of a thyroid hormone, in other situations less. This applies also to thyroid hormones. Therefore, doses of thyroxine need to be adjusted occasionally.

Underdosing levothyroxine

An underdose of thyroxine leads to symptoms like fatigue, concentration problems, weight gain and depressive moods. This can happen if you do not take the medication regularly or not fasting, for example. Therefore, pay close attention to your doctor's instructions and contact him if you are not feeling well despite taking your medicine.

Restlessness, palpitations, nervousness: Are thyroid hormones to blame?

Side effects of thyroid hormones can particularly occur at the beginning of the therapy. The body needs some time to get used to the hormone. In addition, it usually takes a lot of time and patience to find your individual "feel-good dose". It is important to start the treatment as low as possible and then slowly increase the dose so that the body has enough time to get used to the hormones. Some typical symptoms of an overdose include restlessness, heart racing and nervousness, among others mentioned earlier (see side effects).

If you notice any side effects while taking L-thyroxine, you should consult your doctor. He will check your thyroid hormone including the so-called TSH regularly, which gives you information about the function of the thyroid gland. A high TSH can indicate an underactive thyroid gland. A low TSH, on the other hand, can indicate a thyroid hyperactivity.

Is lifelong intake of thyroxin dangerous?

No. Anyone who is dependent on taking thyroid hormones (thyroxine) for a lifetime does not have to expect any long-term damage.

Hormone pills are identical to the "real" hormone

The intake of hormone pills is vital and the only way to compensate for the hormone deficiency in cases of hypothyroidism or a lack of thyroid gland.

The chemical composition of the tablets is identical to the body's own hormone T4 (thyroxine). This means that the synthetically produced hormones do not differ from the natural thyroid hormone in terms of their effect. Thus, they are not harmful but helpful to the body.

Adapting your dose slowly

When taking L-thyroxine, however, there are some things to consider. As mentioned before, the body must be accustomed slowly to the artificially supplied hormones in the beginning. Only in the course of time can the dose be increased slowly until the optimal daily amount is reached. An inadvertent intake of large amounts of hormones, on the other hand, can trigger typical symptoms of hyperthyroidism and overdosage.

Side effects occurring in the beginning

L-Thyroxine is usually considered to be a very well tolerated drug and is often used to treat hypothyroidism. As with almost any medicine, side effects (such as palpitations, tremors, hot flushes, restlessness) can occur, especially at the start of treatment, but they usually disappear after a short period of time. Preparations should never be discontinued abruptly, as otherwise the entire hormone balance may be mixed up and further side effects may result.

Conclusion: The lifelong use of L-Thyroxine certainly does not pose any danger to our health. Rather, the body is dependent on the regular supply of the medication in order to function properly.

How to avoid an overdose

In order to avoid overdosage, you should not take a missed dose of this medicine more often but continue to take it at regular intervals. It is also advisable to have your thyroid values checked regularly by your doctor to confirm the correct dose, especially at the beginning of treatment, when adjusting your dose or switching to another preparation and during pregnancy.

The TSH-level is of importance. This is the laboratory value that determines the so-called thyroid-stimulating hormone (TSH) and controls the body's own production of thyroxine.

When should L-Thyroxine not be taken?

The following conditions or diseases can be contraindications against levothyroxine (L-thyroxine):

  • Untreated hyperthyroidism
  • Hypersensitivity to L-thyroxine or any other component of the preparation
  • Untreated insufficiency of the adrenal gland
  • Untreated insufficiency of the pituitary gland (pituitary insufficiency)
  • Acute heart attack
  • Acute myocarditis

Other situations in which thyroxine should not be taken

Before starting levothyroxine, high blood pressure, coronary artery disease (CHD) and uncontrolled thyroid hormone production (thyroid autonomy) must also be excluded or treated. Apart from thyroid autonomy, this also applies to the performance of a thyroid function test, in which thyroxine must also be taken.

How do you treat a levothyroxine overdose?

Overdosing levothyroxine is not that uncommon. In may particularly occur at the beginning of treating hypothyroidism, when starting levothyroxine without yet knowing exactly the individual dose needed. If you experience any discomfort under your thyroxine dose, talk to your doctor.

An excess of thyroid hormones can manifest itself in restlessness, heart racing with a fast heartbeat, sleep problems and diarrhoea, for example. These are typical symptoms of hyperthyroidism. If you notice these symptoms, you should consult your doctor as soon as possible and discuss further treatment steps. He will draw your blood and determine your thyroid gland values. One aspect of those levels, an elevated T3, is an indicator of an overdose.

If necessary, your doctor may reduce your dose of L-thyroxine for a while or stop it altogether until your blood counts return to normal and you are feeling well again. After another check-up the treatment can be continued carefully.

Don´t worry!

If you have taken too much thyroid hormones, there is usually no reason to worry too much. Severe problems like the so-called thyrotoxic crisis, which can be life-threatening, only occur rarely.

Also: underdosing can be just as harmful. Therefore, it is important to give yourself some time, find the right dose and take your pills regularly as prescribed by your doctor.

Effects on weight and pregnancy

How do thyroid hormones affect weight?

In patients with hypothyroidism, the metabolism slows down, the energy turnover decreases and the weight subsequently increases. By replacing the missing hormone, you can usually counteract the weight gain.

Despite eating the same amount of food and calories, the weight keeps increasing in these patients, just because of the reduced metabolism. Yet, it is impossible to predict how much weight you will gain. This also depends on your overall athletic activity and the general eating behaviour.

Misuse of thyroid hormones

Replacement of missing thyroid hormones can help to lose weight. Regular use of the medication slowly normalizes the hormonal balance. After a while of taking your medicine regularly, your metabolism is revived, and the weight gain due to hypothyroidism goes back towards normal levels.

Yet, it may take a while, before the weight reducing effect of the thyroid hormone replacement sets in. Hormone replacement therapy therefore always requires a lot of patience from both the patient and the doctor. In rare cases the appetite can be increased by taking thyroid pills. If this is the case, you should consult your doctor.

Can I get pregnant when taking levothyroxine?

Yes, you can. Yet, your thyroid levels must be checked regularly. Proper thyroid levels are essential for pregnancy and lactation time. Due to your hormonal changes during pregnancy, your thyroxine dose may have to be adjusted by your doctor.

Authors: Eva Bauer (doctor), Dr. Hubertus Glaser, Nina Schratt-Peterz & Dr. med. Jörg Zorn


  • Red list, patient information service: L-Thyroxine, online at (last accessed 23.09.2019).
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