L-Thyroxine Christiaens (Anhydrous Levothyroxine Sodium)

Levothyroxine is used to replace deficient thyroid hormone in several thyroid‑related conditions.

• Hypothyroidism: restores normal metabolic rate and alleviates symptoms such as fatigue and weight gain.
• Goiter: reduces thyroid gland enlargement by normalizing hormone levels.
• Thyroid cancer (adjunct therapy): provides hormone replacement after thyroidectomy to maintain metabolic homeostasis.

• Levothyroxine is a synthetic form of the endogenous thyroid hormone thyroxine (T4). After oral absorption, it enters the bloodstream and is converted to the active hormone triiodothyronine (T3) within target cells. Both T4 and T3 bind to nuclear thyroid hormone receptors, modulating gene transcription that controls basal metabolic rate, protein synthesis, and growth. The regulated increase in intracellular T3 levels ensures appropriate physiological responses across multiple organ systems.

Levothyroxine may cause a range of adverse reactions, varying from mild to severe.

Common Side Effects

• Headache: mild headache reported in some patients.
• Insomnia: difficulty falling or staying asleep.
• Palpitations: sensation of rapid or irregular heartbeat.
• Weight loss: unintended loss of body weight.

Serious or Rare Side Effects

• Cardiac arrhythmia: potentially life‑threatening irregular heart rhythm.
• Angioedema: swelling of the face, lips, tongue, or throat.
• Anaphylaxis: severe allergic reaction requiring immediate medical attention.

Levothyroxine therapy requires careful monitoring and individualized dosing.

• Dose adjustment: titrate dose based on thyroid function tests and clinical response.
• Cardiovascular disease: use caution in patients with coronary artery disease or arrhythmias.
• Pregnancy: monitor thyroid levels closely and adjust dose as needed.
• Elderly: start with lower dose to avoid overtreatment.
• Drug interactions: review concomitant medications that may affect levothyroxine absorption.
• Store at ambient temperature: keep below 25 °C, in original container, protected from moisture and light.

Levothyroxine absorption and activity can be altered by several concomitant agents.

Major Interactions (Avoid)

• Calcium supplements: markedly reduce levothyroxine absorption when taken simultaneously.
• Iron salts: significantly impair absorption if co‑administered.
• Antacids containing aluminum or magnesium: decrease bioavailability.
• Cholestyramine: binds levothyroxine and reduces its effect.

Moderate Interactions (Monitor Closely)

• Warfarin: levothyroxine may enhance anticoagulant effect, requiring INR monitoring.
• Oral contraceptives: may increase levothyroxine binding, necessitating dose adjustment.
• Rifampin: accelerates metabolism, potentially lowering serum levels.