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Levothyroxine: Effects, Dosage & Side Effects

Levothyroxine is a synthetic thyroid hormone used to treat hypothyroidism. It replaces the naturally produced hormone thyroxine in the body.

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Things worth knowing about "Levothyroxine"

Levothyroxine is a synthetic thyroid hormone used to treat hypothyroidism. It replaces the naturally produced hormone thyroxine in the body.

What is Levothyroxine?

Levothyroxine (also known as L-thyroxine or T4) is a synthetically produced thyroid hormone that is chemically identical to the hormone thyroxine naturally produced by the body. It is used in medicine to compensate for a deficiency of thyroid hormones or to support the treatment of thyroid tumors. Levothyroxine is one of the most commonly prescribed medications worldwide.

Indications

Levothyroxine is used for the following conditions:

Hypothyroidism (underactive thyroid): The most common indication. The thyroid gland does not produce enough thyroxine, leading to symptoms such as fatigue, weight gain, and sensitivity to cold.
Hashimoto thyroiditis: An autoimmune condition in which the immune system attacks the thyroid gland and impairs its function.
Goiter: Enlargement of the thyroid gland, where levothyroxine can help inhibit further growth.
After thyroid removal (thyroidectomy): To fully replace the missing hormone production.
Thyroid cancer: In higher doses to suppress TSH levels (suppression therapy), thereby inhibiting tumor growth.

Mechanism of Action

Levothyroxine is the synthetic form of the thyroid hormone thyroxine (T4). After ingestion, T4 is converted in the body's cells by the enzyme deiodinase into the biologically active form triiodothyronine (T3). T3 binds to nuclear receptors within cells and thereby influences gene expression. This regulates numerous vital metabolic processes, including:

Energy metabolism and basal metabolic rate
Heart rate and cardiovascular function
Body temperature regulation
Growth and development (especially in children)
Nervous system function and cognitive performance

Dosage

The dosage of levothyroxine is individual and is determined by the treating physician based on blood tests, particularly the TSH level. Typical doses range between 25 and 200 micrograms per day. Levothyroxine should generally be taken in the morning on an empty stomach, approximately 30 minutes before breakfast, to ensure optimal absorption.

Important Notes on Administration

The tablet should be swallowed with sufficient water.
Certain foods (e.g., soy products, coffee) and medications (e.g., calcium or iron supplements) can inhibit the absorption of levothyroxine and should be taken at a different time.
The dose must not be changed without medical advice.

Side Effects

At a correctly adjusted dose, side effects are rare. However, an overdose can cause symptoms of hyperthyroidism (overactive thyroid):

Palpitations and cardiac arrhythmias
Restlessness, nervousness, and insomnia
Weight loss despite normal appetite
Sweating and heat intolerance
Hand tremors
With long-term overdosage: increased risk of osteoporosis and cardiac arrhythmias (e.g., atrial fibrillation)

Drug Interactions

Levothyroxine can interact with various medications and substances:

Antacids, calcium, and iron supplements: Reduce the absorption of levothyroxine.
Blood thinners (e.g., warfarin): Levothyroxine can enhance their effect.
Antidiabetic medications: Insulin requirements may change.
Certain antidepressants (e.g., sertraline): May increase levothyroxine requirements.

Special Patient Groups

Pregnancy

During pregnancy, the need for thyroid hormones increases significantly. A well-adjusted levothyroxine therapy is essential for the healthy development of the child. Pregnant women should have their TSH levels checked regularly and adjust the dose as needed.

Children and Adolescents

Levothyroxine is also approved for children, particularly in cases of congenital hypothyroidism (prevention of cretinism). Early treatment is crucial for normal physical and mental development.

Older Patients

In older patients, therapy is initiated at a lower dose and increased slowly to avoid placing excessive strain on the cardiovascular system.

References

Garber, J.R. et al. - Clinical Practice Guidelines for Hypothyroidism in Adults. American Thyroid Association (ATA), 2012. Thyroid, 22(12):1200-1235.
World Health Organization (WHO) - Model Formulary 2008: Thyroid Hormones. WHO Press, Geneva.
European Medicines Agency (EMA) - Summary of Product Characteristics for Levothyroxine Sodium, current valid version.

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