Letter (Levothyroxine Sodium)

Levothyroxine sodium is used in thyroid hormone replacement therapy to manage conditions caused by insufficient endogenous hormone.

• Hypothyroidism: restores normal metabolic activity by providing exogenous thyroxine (T4).
• Post‑thyroidectomy hormone replacement: maintains euthyroid status after surgical removal of the thyroid gland.
• Congenital hypothyroidism: supplies necessary thyroid hormone to infants born with thyroid deficiency.

• Levothyroxine sodium is a synthetic form of the natural thyroid hormone thyroxine (T4). After oral absorption, it is converted to the active hormone triiodothyronine (T3) in peripheral tissues, binding to nuclear thyroid hormone receptors and regulating gene transcription that controls basal metabolic rate, protein synthesis, and growth.

Levothyroxine sodium may cause a range of adverse reactions, most of which are dose‑related.

Common Side Effects

• Headache: mild to moderate intensity.
• Insomnia: difficulty falling or staying asleep.
• Palpitations: sensation of rapid or irregular heartbeat.
• Weight loss: unintended reduction in body weight.
• Heat intolerance: increased sensitivity to warm environments.
• Tremor: fine shaking of hands or limbs.

Serious or Rare Side Effects

• Cardiac arrhythmia: irregular heart rhythm that may be life‑threatening.
• Angina pectoris: chest pain due to reduced coronary blood flow.
• Myocardial infarction: heart attack in susceptible individuals.
• Bone loss/osteoporosis: decreased bone mineral density with long‑term overtreatment.
• Severe allergic reaction: rash, swelling, or anaphylaxis.
• Adrenal crisis: precipitated in patients with adrenal insufficiency.

Levothyroxine sodium requires careful monitoring to avoid over‑ or under‑replacement.

• Dose titration: adjust based on thyroid function tests and clinical response.
• Cardiovascular disease: use cautiously in patients with angina, arrhythmias, or recent myocardial infarction.
• Pregnancy and lactation: consult a healthcare professional; dose may need adjustment.
• Elderly patients: start with lower doses and monitor for cardiac effects.
• Drug interactions: review concomitant medications that may affect absorption or metabolism.
• Store at ambient temperature: keep below 25°C, retain in original container, protect from moisture and direct sunlight.

Levothyroxine sodium can have significant interactions that affect its absorption or hormonal activity.

Major Interactions (Avoid)

• Calcium supplements: reduce levothyroxine absorption.
• Iron salts: markedly decrease oral bioavailability.
• Sucralfate: binds and impairs absorption.
• Cholestyramine or other bile‑acid sequestrants: lower systemic exposure.
• Antacids containing aluminum or magnesium: interfere with uptake.

Moderate Interactions (Monitor Closely)

• Warfarin: may enhance anticoagulant effect, requiring INR monitoring.
• Diabetes medications: thyroid hormone changes can affect glucose control.
• Antidepressants (SSRIs, tricyclics): may alter thyroid hormone levels.
• Estrogen therapy: can increase levothyroxine requirement.
• Statins: potential modest effect on thyroid hormone metabolism.