L-Thyroxin Bc (Levothyroxine Sodium, Hydrate)

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

• Hypothyroidism: normalizes thyroid hormone levels, relieving fatigue, weight gain, and cold intolerance.
• Post‑thyroidectomy replacement: provides necessary thyroxine after surgical removal of the thyroid gland.
• Goiter: reduces thyroid‑stimulating hormone drive, helping to shrink an enlarged thyroid.

• 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 receptors, modulating gene transcription that controls basal metabolic rate, protein synthesis, and growth. By restoring circulating T4, it re‑establishes feedback inhibition of the pituitary, normalizing TSH secretion and thereby improves clinical symptoms associated with deficiency.

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

Common Side Effects

• Headache: mild to moderate headache.
• 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: abnormal heart rhythm that may be life‑threatening.
• Angina pectoris: chest pain due to reduced coronary blood flow.
• Myocardial infarction: heart attack in susceptible patients.
• Bone loss: accelerated osteoporosis, especially with long‑term use.
• Severe allergic reaction: rash, swelling, or anaphylaxis.
• Adrenal crisis: acute adrenal insufficiency precipitated by excess thyroid hormone.

When prescribing Levothyroxine, clinicians should consider several precautionary measures to ensure safe use.

• Cardiovascular disease: start with low dose and monitor heart rate.
• Elderly patients: assess for frailty and adjust dose cautiously.
• Adrenal insufficiency: ensure adequate glucocorticoid replacement before initiating therapy.
• Pregnancy and lactation: evaluate risks and benefits; adjust dose based on thyroid function tests.
• Drug monitoring: regularly check serum TSH and free T4 levels to guide dosing.
• Interference with absorption: separate administration from calcium, iron, or antacid products by at least 4 hours.
• Store at ambient temperature: keep in the original pack, protect from moisture and light.

Levothyroxine interacts with several medicines that can alter its absorption or hormonal activity.

Major Interactions (Avoid)

• Calcium supplements: form insoluble complexes, markedly reducing levothyroxine absorption.
• Iron salts: decrease oral bioavailability when taken concurrently.
• Cholestyramine or colestipol: bind the drug in the gut, preventing uptake.

Moderate Interactions (Monitor Closely)

• Antiepileptic drugs (phenytoin, carbamazepine, phenobarbital): increase metabolism, possibly lowering hormone levels.
• Rifampin: induces hepatic enzymes, enhancing clearance of levothyroxine.
• Estrogen therapy (oral contraceptives, hormone replacement): may raise thyroxine‑binding globulin, requiring dose adjustment.
• Warfarin: levothyroxine can potentiate anticoagulant effect; monitor INR closely.