L-thyroxine (syn. Levothyroxine, T4) is a synthetically produced thyroid hormone. It replaces the thyroxine (T4) present in the human body, which is a precursor of the second thyroid hormone triiodothyronine (T3). The thyroid hormones are important for the development and function of the entire organism.

They are mainly needed for the maturation of the brain. Among other things, they influence the nervous system and psyche and the growth of skin, hair and nails. In addition, they play an essential role in the energy metabolism, they also control the function of the stomach and intestines, heart and circulation.

A drug that contains L-thyroxine, for example, is Thyronajod®.

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L-thyroxine is used to treat the Hypothyroidism (Hypothyroidism) required. People with this disease often have the following symptoms:

  • Weight gain
  • constant fatigue
  • Sensitivity to cold
  • Poor memory
  • depressive moods
  • Muscle weakness
  • Cool, pale, and dry skin
  • Brittle, brittle hair up to increased hair loss
  • Brittle nails
  • Chronic constipation
  • Goiter (goiter)
  • Menstrual cycle disorders in women
  • Decrease in sexual pleasure and potency in men
  • Swelling of the subcutaneous fat (so-called myxedema)

These symptoms are not seen in all patients with hypothyroidism. In particular elderly often show only a few typical characteristics this disease.

Treating hypothyroidism involves making up for the lack of thyroid hormone, thereby reducing symptoms.

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In addition, the hormones with this therapy are also after Removal of the thyroid as part of a Thyroid cancer and also to reinforce the classic Antidepressants in the treatment of depressions used.

The thyroid hormones are supplied to the human body in tablet form. In most cases, this treatment must be carried out for a lifetime.


L-thyroxine has the same structure as an endogenous thyroid hormone. Therefore, it works in the same way as thyroid hormones. The main effect is to increase the body's drive and activity. This happens because L-thyroxine activates signaling pathways inside the cells that activate the metabolism. For example, the breakdown of fats and carbohydrates is stimulated in order to provide energy. As a result, the basal metabolic rate increases when taking L-thyroxine. L-thyroxine also increases the activity of the heart. The signaling pathways triggered by L-thyroxine lead to increased incorporation of so-called beta receptors. These receptors increase the strength and rate of the heart when activated.
In children, thyroid hormones and, if necessary, L-thyroxine are important for growth and maturation. For example, they promote the development processes in nerves and bones. When taking L-thyroxine for the first time, the effect occurs after one to three weeks. Until then, the amount of L-thyroxine in the blood will slowly increase when you take it. The body can adapt its functions and metabolism to the new hormone levels. In order to maintain the effect permanently, L-thyroxine must be taken regularly in the dosage prescribed by the doctor.

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This article provides information on other drugs used as a therapy for hypothyroidism: Thyroid medication

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Taking L-thyroxine

Tablets containing the synthetically produced thyroxine (T4), which corresponds to the human thyroid hormone thyroxine, are taken. L-thyroxine is preferred to the other thyroid hormone T3 because it stays in the body for a longer time. T3 is then automatically produced in the body from T4 in the required amount.

It should be taken 30 minutes before meals so as not to reduce absorption in the intestines through food.

The therapy is started gradually, which means it is started with a low dose (mostly 25-50 µg) and then slowly increased (mostly 75-200 µg). The individual correct dose is determined by sneaking in and the accompanying blood tests. The need for L-thyroxine depends on the age of the patient and decreases with age. The need can also vary greatly from person to person.

If taken correctly, those affected can lead a completely normal life. Blood tests will be done about once a month at the start of treatment. When the levels of hormones in the blood return to normal, the test is done every three to six months.

Because of the long half-life of L-thyroxine of 190 hours, there are no significant consequences if the intake is forgotten. Therapy will then be continued as usual the following day.

Read more on the subject at: T3-T4 hormones


L-thyroxine fulfills the same functions as the body's own thyroid hormones. As a result, L-thyroxine is used when the thyroid gland no longer produces enough hormones on its own. The amount of hormones that is not produced enough must therefore be replaced by the corresponding amount of L-thyroxine. For this reason, the dosage of L-thyroxine must be individually adjusted based on the type and severity of the disease. In addition, the amount of hormones required by the body and thus the dose of L-thyroxine varies depending on the age, height and weight of the patient.
To find out the amount of L-thyroxine required, a blood test by a doctor is necessary. This must be repeated during the course of therapy as a control examination. In this way, the doctor can always precisely determine and adjust the dosage for the respective patient. Once this has happened, the full dose should be taken one day on an empty stomach, at least half an hour before the first meal.
In general, a dose of 25-50 micrograms per day is started. This is then increased step by step and increased by 25 - 50 micrograms each within a month. This continues until the appropriate dosage is reached. If the thyroid is congenitally underactive, the average is 100-200 micrograms. The same amount is used for Hashimoto's thyroiditis and usually has to be taken until the end of life. A hypofunction can also only arise through interventions or treatments of the thyroid gland. If the thyroid gland has been completely removed, for example due to a tumor, the body can no longer produce thyroid hormones at all. In this case, the full amount of hormones the body needs must be replaced with L-thyroxine. A dosage of up to 300 micrograms per day is used for this. This is usually administered for life. If only part of the thyroid is diseased, for example due to a benign lump, the maximum dose required is 200 micrograms per day. If this regresses, the drug can in some cases be discontinued after six months to a few years.

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There are also drugs that reduce the function of the thyroid gland. They are called anti-thyroid drugs and are used to treat an overactive thyroid. Although it seems paradoxical at first, a small amount of L-thyroxine can also be used in this therapy. In this way, the optimal hormone level in the blood can be precisely adjusted, even when the thyroid is slowed down. A dose of 50-100 micrograms per day is usually prescribed here. This is taken as long as the anti-thyroid drugs.
When taking L-thyroxine during exercise, the dosage does not need to be adjusted. It should be noted that taking L-thyroxine increases the drive of the metabolism and the heart. This can make training easier. However, it should be noted that L-thyroxine can only increase the body's performance if there is a deficiency in thyroid hormones. If the hormone level is normal, the metabolism is set in motion by taking other medication to such an extent that proteins in muscles are broken down and cardiac arrhythmias can occur.
Furthermore, the correct dosage of L-thyroxine is very important during pregnancy. With the correct dosage, there are no known side effects that could harm the child. During pregnancy, the increased amount of hormones such as estrogen in the mother's blood also increases the amount of thyroid hormones required. For this reason, the doctor should monitor the hormones in the blood even more closely during pregnancy. Thus, the dosage can always be precisely adjusted and the child can be protected.

What should be considered when stopping?

The effect of L-thyroxine affects the body in many ways and has a particular influence on the metabolism and heart rhythm. The body adapts to the new thyroid hormone level during treatment. Therefore, suddenly stopping L-thyroxine can be very dangerous. The dose may therefore not be reduced without consulting a doctor.
If serious side effects occur, an adjustment of the dosage should be discussed with the doctor before the next medication is taken. In addition, the drug must be taken regularly in order to keep the amount of thyroid hormones in the blood constant and thus achieve the desired effect through treatment with L-thyroxine. Therefore, L-thyroxine must not be discontinued even if the symptoms improve.

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Side effects

When taking L-thyroxine, especially if the dose is increased too quickly at the beginning of treatment, typical symptoms may occur Hyperthyroidism occur with the following symptoms:

  • Palpitations
  • Cardiac arrhythmias
  • Chest tightness and pain
  • Increase in blood pressure
  • Feeling hot
  • Tremor (tremor)
  • inner unrest
  • Diarrhea, weight loss, increased appetite
  • a headache

As atypical phenomena can also fever and Vomit occur.

Also one Hypersensitivity against L-thyroxine can occur, it comes to allergic reactions on the skin and in the respiratory tract.


L-thyroxine has the same effect in the body as the body's own thyroid hormones. Among other things, these activate the work of the digestive tract. This promotes the movement of the intestine and thus the further pushing of the food. If the thyroid hormones are too high or in an unaccustomed amount for the body, the intestinal drive can be increased to such an extent that diarrhea occurs. For this reason, this side effect can occur even with the correct dosage when starting L-thyroxine.

Weight loss

An essential effect of the thyroid hormones and thus also of L-thyroxine is the increase in the body's willingness to be active and the associated increase in body temperature. The entire metabolism is set in motion in order to provide energy. Therefore, the body burns more calories and reduces reserves. Because of this, taking L-thyroxine can result in weight loss. Nevertheless, the drug should not be misused for this purpose, especially in people with healthy thyroid glands. The reason for this is that the increase in activity primarily affects the heart and can lead to consequences ranging from heart stumbling to sudden cardiac death.

Muscle aches

L-thyroxine increases the body's readiness for activity and promotes metabolic processes. In order to provide enough energy, more carbohydrates and fats are burned. So that the body has enough strength to become active, low doses of L-thyroxine lead to the development of the muscle. If the dose of L-thyroxine is very high, however, this means that the body has to provide even more energy. Energy reserves in the muscle can then also be used for this.This is broken down and the released muscle proteins can now be used as energy suppliers. This can manifest itself in the form of muscle pain as a side effect of L-thyroxine. For this reason, muscle pain that occurs for no obvious cause, such as excessive exercise, should be discussed with a doctor. If necessary, he or she can adjust the dosage of L-thyroxine.

Loss of libido

Loss of libido is not one of the side effects known for L-thyroxine. However, there may be a connection between the amount of thyroid hormones in the blood and the desire for sex. If the thyroid is underactive, the body's drive is severely restricted and people affected often feel slowed down. Occasionally, this can be accompanied by a loss of libido, which the right hormone levels can improve. In addition, too high a dosage of L-thyroxine should also be avoided. This can lead to restlessness, cardiac arrhythmias and circulatory problems and thus also influence sexual desire.

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L-thyroxine must not be used if a Hyperthyroidism present. In addition, you should refrain from taking the drug if the following diseases cannot be ruled out:

  • Coronary artery diseases (e.g. Angina pectoris)
  • high blood pressure
  • Weakness of the pituitary gland (pituitary insufficiency) or the adrenal cortex (adrenal insufficiency)
  • Acute Heart attack
  • Myocarditis ( Myocarditis)
  • Cardiac arrhythmias ( Tachyarrhythmias)

When treating women after the Menopausewho have an elevated Risk of osteoporosis and have an underactive thyroid, the thyroid should be regular controlled be to increased values to prevent L-thyroxine in the blood.

high blood pressure

If you have existing and untreated high blood pressure, L-thyroxine must not be taken. The reason for this is that L-thyroxine has the effect of increasing the body's drive and readiness for activity. This effect is accompanied by an increase in blood pressure. This is very dangerous if you have high blood pressure. Consequently, blood pressure should be checked before starting L-thyroxine. In this way, existing high blood pressure can be recognized and treated if necessary. Once this has happened, L-thyroxine may be taken.


L-thyroxine reduced the effect of blood sugar lowering drugs (Antidiabetic drugs). Therefore the Blood sugar level especially at the beginning of thyroid hormone therapy in diabetic patients (diabetic) are checked regularly. If necessary, the dose the blood sugar lowering drug.

L-thyroxine can cause certain effects anticoagulants (Coumarin derivatives) amplify. Therefore it is recommended to use the Blood clotting values to have regular control during therapy with L-thyroxine.

Barbiturates (certain Sleeping pills) and certain other medicines can cause the Dismantling of L-thyroxine through the liver accelerate and thus a decreased effect of L-thyroxine.

During one pregnancy In patients with hypothyroidism, the need for L-thyroxine can be increased by the increased blood level female sex hormones (especially Estrogen) be increased.
Thyroid function should therefore be monitored both during and after pregnancy and the dose of thyroid hormone adjusted if necessary.

From one simultaneous use of L-thyroxine and anti-thyroid drugs (Anti-thyroid drugs) is strongly discouraged as this would require a higher amount of anti-thyroid drugs. Anti-thyroid drugs are capable of (in contrast to L-thyroxine) over the placenta to get into the child's circulation and can cause a Hypothyroidism cause.

L-thyroxine and the pill - are they compatible?

As with many drugs, there can also be interactions between L-thyroxine and the pill. This particularly affects pills that contain estrogens as an active ingredient. They can reduce the effect of L-thyroxine. Therefore, when taking the pill for the first time or when discontinuing the pill, the dosage of L-thyroxine must be discussed with the doctor, checked and, if necessary, adjusted. The other way around, however, it is not known that L-thyroxine affects the effectiveness of the pill. Therefore, contraceptive protection is still given when taking it at the same time.

Is L-thyroxine available without a prescription?

Since L-thyroxine has great effects on the heart, metabolism and circulation, L-thyroxine cannot be bought without a prescription. The correct dosage is very important to avoid serious side effects. This can only be determined by a doctor using a blood sample and must continue to be checked during the course of treatment.

L-thyroxine abuse

Thyroxine is also used as a drug for abuse Weight loss used. There is neither approval nor proof of effectiveness for this indication. Admittedly, the intake of thyroid hormone can Energy turnover increase, but what by a increased food intake can be balanced. Also Deaths after overdose are known.

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