L-Thyroxine Christiaens (Anhydrous Levothyroxine Sodium)

Levothyroxine is a thyroid hormone replacement that acts on the hypothalamic‑pituitary‑thyroid axis to restore normal hormone levels.

• Hypothyroidism: replaces deficient thyroid hormone, normalizing metabolic rate, energy levels, mood, and cardiovascular function.
• Goiter: lowers elevated TSH stimulation, allowing gradual reduction of thyroid gland size and alleviating compressive symptoms.
• Thyroid cancer suppression: provides exogenous thyroxine to suppress TSH, reducing stimulation of residual thyroid tissue and decreasing risk of tumor recurrence.

• Levothyroxine is a synthetic form of the endogenous hormone thyroxine (T4). After oral absorption, it is converted to the active hormone triiodothyronine (T3) in peripheral tissues. The hormone binds nuclear thyroid hormone receptors, modulating gene transcription that regulates basal metabolic rate, protein synthesis, and sympathetic nervous system activity. By restoring circulating T4 levels, levothyroxine normalizes feedback inhibition of the hypothalamic‑pituitary‑thyroid axis, reducing thyroid‑stimulating hormone secretion.

Levothyroxine may cause a range of adverse reactions, from mild, transient symptoms to potentially serious events that require prompt medical attention.

Common Side Effects

• Headache: mild to moderate headache, often transient.
• Insomnia: difficulty falling or staying asleep.
• Palpitations: sensation of rapid or irregular heartbeat.
• Weight loss: unintended loss of body weight, especially at higher doses.

Serious or Rare Side Effects

• Cardiac arrhythmia: abnormal heart rhythm that may be life‑threatening.
• Angina pectoris: chest pain due to reduced coronary blood flow.
• Anaphylaxis: rare severe allergic reaction with rash, swelling, and breathing difficulty.

When using levothyroxine, consider the following precautions to ensure safe and effective therapy.

• Dose titration: adjust dose gradually, especially in elderly or cardiac patients.
• Cardiovascular disease: monitor for angina or arrhythmias when initiating therapy.
• Adrenal insufficiency: ensure adequate glucocorticoid replacement before starting levothyroxine.
• Pregnancy and lactation: use only if benefits outweigh risks; monitor thyroid function closely.
• Thyroid function monitoring: check TSH and free T4 levels regularly to maintain target range.
• Store at ambient temperature: keep tablets below 25 °C in the original container, protect from moisture and light.

Levothyroxine can interact with several medicines, affecting its absorption or efficacy.

Major Interactions (Avoid)

• Antacids containing aluminum or magnesium: markedly reduce levothyroxine absorption.
• Sucralfate: binds levothyroxine and prevents its uptake.
• Iron supplements: significantly decrease oral levothyroxine bioavailability.

Moderate Interactions (Monitor Closely)

• Estrogen therapy: may increase levothyroxine requirement by raising thyroid‑binding globulin.
• Carbamazepine and phenytoin: accelerate levothyroxine metabolism, potentially lowering serum levels.
• Rifampin: induces hepatic enzymes, reducing levothyroxine effectiveness.
• Warfarin: levothyroxine may enhance anticoagulant effect; monitor INR closely.