Introduction and mode of action

The active ingredient in the drug Euthyrox® from Merck Pharma GmbH is called levothyroxine.
Euthyrox® contains the synthetically produced thyroid hormone levothyroxine (L-thyroxine). This is used for diseases of the thyroid gland (e.g. underactive thyroid = hypothyroidism).
In healthy people, the thyroid gland produces various hormones, including the thyroxine, produces. These hormones are required for many metabolic processes and thus control body growth, protein and fat metabolism, among other things. If hypothyroidism is present, too few or no thyroid hormones are produced by the body. These must therefore be replaced artificially. In this case, the synthetic levothyroxine is often prescribed. This synthetic hormone has the same effect as thyroxine (T4) and is then partially converted by the body into the thyroid hormone (T3).

Euthyrox® is available in the form of tablets in dose strengths of 25 - 200 µg. It is used for benign enlargements of the thyroid gland (so-called goiter), provided that the organ function is normal. It is also common to use it after the operation of a goiter to prevent further goiter formation.Euthyrox® is also used for hypothyroidism (hypothyroidism) and malignant thyroid tumors (thyroid malignancy). The therapy of an overactive thyroid gland (hyperthyroidism) using Euthyrox® is also known, provided that the patient receives anti-thyroid drugs (anti-thyroid drugs). The use of Euthyrox® is also common in the so-called thyroid suppression test, in which certain amounts of levothyroxine are administered over a few days in order to determine whether the thyroid gland has an independent hormone metabolism that is independent of the control of the pituitary gland.


The dose of the drug Euthyrox® is individually adjusted to the needs of each individual patient.

The entire daily dose prescribed by the doctor is taken in the morning with a glass of water on an empty stomach. Then nothing should be eaten for at least half an hour.
In most cases, Euthyrox® has to be taken for a lifetime, as the hypothyroidism or the lack of the thyroid gland (e.g. after surgery) cannot be compensated for other than by substituting the hormones in a synthetic way.

With a benign enlargement of the thyroid gland, but normal organ function, Euthyrox® is actually only taken temporarily.

Are the Euthyrox® tablets divisible?

The tablets are notched for dividing. This allows the tablet to be easily halved if the dose of a whole tablet is too high. Euthyrox tablets are available from 25 to 200 micrograms.


Before starting treatment with Euthyrox®, the following diseases must have been excluded or treated:

  • Coronary heart disease (CHD)
  • Tightness of the heart (angina pectoris)
  • arteriosclerosis
  • High blood pressure (hypertension)
  • Hypofunction of the pituitary gland (pituitary insufficiency)
  • Underactive adrenal cortex (adrenal insufficiency)
  • Thyroid autonomy

Patients with Euthyrox® are unsuitable for treatment with

  • Hypersensitivity to components of the drug
  • untreated overactive thyroid gland (hyperthyroidism)
  • untreated underactive adrenal glands (adrenal insufficiency)
  • untreated subfunction of the pituitary gland (pituitary insufficiency)
  • more acute Heart attack
  • acute Inflammation of the heart muscle (myocarditis)
  • acute inflammation of the heart wall (pancarditis)

Side effects

If Euthyrox® is used in a controlled manner, no side effects are to be expected, as it is a replacement for missing body hormones.

However, if the prescribed dose of Euthyrox® is not tolerated or if there is an overdose, symptoms that resemble an overactive thyroid may occur. These include the following symptoms:

  • Racing heart
  • Cardiac arrhythmia
  • Tremble
  • inner unrest
  • insomnia
  • increased sweating
  • Feeling hot
  • fever
  • Weight loss
  • Weight gain
  • Vomit
  • a headache
  • Muscle spasms
  • Menstrual irregularities
  • diarrhea

If these symptoms are noticeable, the daily dose must be reduced or, in extreme cases, taking Euthyrox® must be paused for a few days. Once the side effects have disappeared, the use of Euthyrox® can be cautiously resumed with a slow dose increase to a minimum initial dose.

If there is a hypersensitivity to an ingredient in Euthyrox®, allergic reactions of the skin or the respiratory tract can occur.

Weight loss as a side effect

The thyroid hormones are metabolic hormones. They ensure a smooth metabolism and have many functions.
An excess of thyroid hormones can therefore lead to an imbalance in the metabolism. The oversupply can either result from an overdose of Euthyrox®, or from disorders of the thyroid gland such as Graves' disease or Hashimoto's thyroiditis.
An overdose of Euthyrox® can lead to weight loss due to increased fat breakdown, increased protein breakdown and increased carbohydrate breakdown. In addition to the metabolic functions, the thyroid hormones also have functions in the heart, lungs and muscles and are important in children for growth and intellectual development. Therefore, the thyroid hormones should always be kept in balance so that neither an overactive nor an underactive occurs.

Weight gain as a side effect

If there is a shortage of thyroid hormones due to a too low dose of Euthyrox, weight gain can occur. Here fat, carbohydrate and protein metabolism are slowed down and there is an increased storage of substances. In addition, water retention can occur.
Particular attention must be paid to underactive thyroid or too low a dose of Euthyrox®, especially in children. In addition to weight gain, this can lead to growth retardation and mental retardation. A thyroid hormone test is therefore planned for newborns at U2.

Diarrhea as a side effect

Diarrhea when taking Euthyrox® may be due to an incorrect dosage. Diarrhea can be an expression of an overdose of the drug.
Other symptoms of overdose are increased sweating and temperature, palpitations, restlessness and tremors, and unwanted weight loss. If you have diarrhea that lasts for more than a few days, you should see a doctor for a blood test. Here, the throat hormones can be determined and the dosage adjusted if necessary.

Itching as a side effect

Itching of the skin is usually a sign of an allergic reaction. If this occurs when taking Euthyrox, it may be due to an intolerance to one of the components of Euthyrox®.
If itching or other allergic symptoms persist, the product should be changed (thyroid hormones from another manufacturer). It should be noted that absorption in the intestine differs from manufacturer to manufacturer. It is therefore possible that the same dosage of two different preparations has different effects on the body. Blood tests should therefore be carried out at regular intervals when changing preparations in order to optimally adjust the dosage.

Depression or depressed mood as a side effect

Depression or depressive moods can also occur with a long-term underdose of Euthyrox® or with an underactive thyroid. When the Euthyrox dose is adjusted, the depressive moods should disappear again.

Can Euthyroxin® cause hair loss?

The side effect of hair loss is not known with Euthyroxin®. Euthyroxin® has no effects on the skin or the skin appendages.


The lipid lowering agents colestyramine and colestipol reduce the absorption of levothyroxine and for this reason they should only be used 4 - 5 hours after taking Euthyrox®.

Antacids containing aluminum and calcium carbonate, as well as medicinal products containing iron, also reduce the absorption of levothyroxine and should therefore be taken two hours after Euthyrox® at the earliest. Since levothyroxine is partially converted into liothyronine (T3) in the body, restricting this process by taking propylthiouracil, glucocorticoids, beta-blockers and contrast media containing iodine should be avoided. Amiodarone, which is used against cardiac arrhythmias, can cause an overactive but also an underactive thyroid due to its high iodine content, so that caution is advised, especially with nodular goiter (nodular goiter). If a quick injection of phenytoin (against epilepsy) used, this can increase the blood level of free levothyroxine and liothyronine and thus in rare cases lead to cardiac arrhythmias (symptoms of an overactive thyroid). The following active substances can also raise levels of levothyroxine in the blood:

  • Salicylates (anti-inflammatory, antipyretic)
  • Dicumarol (anticoagulant)
  • Clofibrate (lipid-lowering agent, used to treat increased blood lipid levels)
  • high doses of furosemide /Lasix®(Dehydrating agent)
  • i.a.

The effect of levothyroxine (Euthyrox®), however, is reduced by the following drugs:

  • Sertraline (antidepressant)
  • Chloroquine / proguanil (malaria treatment)
  • Babiturate (sleeping pills)
  • Estrogen-based contraceptives / hormone replacement therapy
  • Soy products

Interactions with medication should also be considered in diabetics, as the blood sugar-lowering effect of antidiabetic drugs can be reduced by levothyroxine (Euthyrox®). For this reason, at the beginning of treatment for hypothyroidism in diabetics, the blood sugar level must be checked very regularly and the dosage adjusted accordingly.

Anticoagulant drugs such as coumarin derivatives can also be influenced in their effect by levothyroxine, which can lead to increased anticoagulation. Here, blood coagulation must be carefully checked when Euthyrox® is administered.

Euthyrox and the pill - are they compatible?

So far, no interactions are known between Euthyrox® and the pill. Due to the variety of preparations for contraception (contraception), however, the gynecologist should be informed about taking the Euthyrox® before taking the pill for the first time.
Overdosing the Euthyrox® can lead to cycle irregularities.

Euthyrox during pregnancy and breastfeeding

The drug Euthyrox® can also be used during pregnancy and breastfeeding. There are no known risks for the unborn child or infant with moderate doses of Euthyrox®.

Due to hormones, the need for levothyroxine in women during pregnancy can increase if they suffer from an underactive thyroid. For this reason, the function of the thyroid gland should be checked particularly carefully during and after pregnancy and the dosage adjusted if necessary.
If the pregnant woman has an overactive thyroid gland (hyperthyroidism), the combination therapy with levothyroxine and the so-called thyreostatics (which inhibit thyroid activity) must never be used.
The suppression test with levothyroxine must also not be carried out during pregnancy due to radioactive substances.

Euthyrox and alcohol - are they compatible?

Euthyrox® contains the active ingredient levothyroxine. Levothyroxine is a synthetically produced thyroid hormone. Normally the thyroid hormones are made in the thyroid gland, the thyrocytes. Since Euthyrox® fulfills exactly the same tasks in the body as the body's own hormones, these also occupy the same receptors and are also broken down by the liver and excreted with the bile.
Euthyrox® replaces the tasks of the thyroid hormones. Since it is a substance that occurs naturally in the body, there are no intolerances to alcohol.
The only thing to note is that the Euthyrox® should be swallowed in the morning before eating in order to ensure optimal absorption in the intestine.