L-Thyroxine (Anhydrous Levothyroxine Sodium)

Levothyroxine is a synthetic thyroid hormone used to treat disorders of thyroid hormone deficiency.

• Hypothyroidism: restores normal thyroid hormone levels and alleviates symptoms such as fatigue, weight gain, and cold intolerance.
• Post‑thyroidectomy hormone replacement: provides necessary thyroid hormone after surgical removal of the thyroid gland.
• Goiter due to hypothyroidism: reduces gland enlargement by correcting hormone deficiency.

• 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 growth. By restoring circulating T4 levels, it normalizes the hypothalamic‑pituitary‑thyroid feedback loop.

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

Common Side Effects

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

Serious or Rare Side Effects

• Cardiac arrhythmia: potentially life‑threatening irregular heart rhythm.
• Angina or myocardial infarction: chest pain or heart attack in susceptible patients.
• Severe osteoporosis: accelerated bone loss with long‑term overtreatment.
• Hyperthyroidism symptoms: severe over‑replacement leading to anxiety, sweating, and tachycardia.
• Anaphylactic reaction: rare but possible severe allergic response.
• Acute hepatic injury: very rare liver inflammation.

Levothyroxine therapy requires careful monitoring and individualized dosing.

• Dose titration: adjust based on thyroid function tests and clinical response.
• Cardiovascular disease: monitor for angina or arrhythmias when initiating therapy.
• Elderly patients: start with a lower dose to avoid overtreatment.
• Pregnancy and lactation: use only if benefits outweigh risks; monitor thyroid levels closely.
• Drug interactions: review concomitant medications that affect absorption or metabolism.
• Store at ambient temperature: keep in the original container, protect from moisture and light.

Levothyroxine interacts with several medicines that can affect its absorption or metabolism.

Major Interactions (Avoid)

• Calcium or iron supplements: markedly reduce levothyroxine absorption; separate dosing by at least 4 hours.
• Proton pump inhibitors: may decrease absorption; monitor thyroid function.
• Amiodarone: can alter thyroid hormone metabolism; require close monitoring.
• Warfarin: levothyroxine may enhance anticoagulant effect; monitor INR closely.

Moderate Interactions (Monitor Closely)

• Antiepileptic drugs (phenytoin, carbamazepine): increase levothyroxine metabolism, may require dose adjustment.
• Cholestyramine or other bile‑acid sequestrants: reduce absorption; monitor thyroid function.
• Estrogen therapy: may increase thyroxine binding, altering serum levels.