L-Thyroxin Inject Henning (Levothyroxine)

Levothyroxine injection is used to restore thyroid hormone levels in conditions affecting the hypothalamic‑pituitary‑thyroid axis.

• Hypothyroidism: restores normal thyroid hormone concentrations, alleviating fatigue, weight gain, and cold intolerance.
• Myxedema: reverses severe hypothyroid manifestations, improving mental status and cardiovascular function.
• Thyroid hormone replacement in patients unable to take oral medication: provides necessary T4 via intravenous route when gastrointestinal absorption is compromised.

• Levothyroxine is a synthetic form of thyroxine (T4) that binds to thyroid hormone receptors in target tissues after conversion to the active triiodothyronine (T3) form. By supplementing deficient endogenous hormone, it normalizes basal metabolic rate, protein synthesis, and sympathetic nervous system activity, thereby correcting the physiological deficits of hypothyroidism. The administered T4 is taken up by cells, where deiodinases convert it to T3, the biologically active hormone that regulates gene expression involved in energy metabolism, growth, and development.

Levothyroxine injection may be associated with a range of adverse reactions, varying from mild to serious.

Common Side Effects

• Headache: mild to moderate headache occurring shortly after administration.
• Nausea: transient nausea or upset stomach.
• Injection site pain: local discomfort or erythema at the infusion site.
• Fever: low‑grade fever without infection.
• Dizziness: light‑headedness or vertigo.
• Fatigue: temporary feeling of tiredness.

Serious or Rare Side Effects

• Cardiac arrhythmia: irregular heart rhythm, especially in patients with pre‑existing heart disease.
• Angina pectoris: chest pain due to myocardial ischemia.
• Myocardial infarction: rare occurrence of heart attack.
• Anaphylaxis: severe allergic reaction with hives, swelling, and respiratory distress.
• Hyperthyroidism: signs of over‑replacement such as tremor, heat intolerance.
• Chest pain: unexplained chest discomfort requiring immediate evaluation.

Before using Levothyroxine injection, several precautions should be observed to ensure safe and effective therapy.

• Baseline thyroid function assessment: confirm hypothyroidism and determine appropriate dosing.
• Cardiac disease monitoring: adjust dose cautiously in patients with ischemic heart disease or arrhythmias.
• Elderly patients: start with lower doses and monitor for over‑replacement symptoms.
• Concomitant medications: review drugs that may interfere with levothyroxine absorption or action.
• Hypersensitivity: avoid use in individuals with known allergy to levothyroxine or formulation components.
• Store at 2-8°C: keep refrigerated, protect from light, do not freeze or shake.

Levothyroxine can interact with several medicines, affecting its absorption, metabolism, or therapeutic effect.

Major Interactions (Avoid)

• Amiodarone: may alter thyroid hormone levels and require dose adjustment.
• Calcium or iron supplements: significantly reduce levothyroxine absorption if taken concurrently.
• Warfarin: can modify anticoagulant effect, necessitating INR monitoring.

Moderate Interactions (Monitor Closely)

• Antacids (aluminum, magnesium): may decrease absorption; separate administration by at least 4 hours.
• Beta‑blockers: can mask symptoms of hyperthyroidism, requiring careful clinical assessment.
• Estrogen therapy (oral contraceptives, hormone replacement): may increase levothyroxine requirement.
• Cholestyramine: binds levothyroxine in the gut, reducing its bioavailability.