Letrox (Levothyroxine Sodium)

Letrox (levothyroxine sodium) is used in thyroid hormone replacement therapy to normalize metabolic function.

• Hypothyroidism: restores normal thyroid hormone levels and alleviates symptoms such as fatigue, weight gain and cold intolerance.
• Post‑thyroidectomy replacement: provides the necessary hormone after surgical removal of the thyroid gland.
• Congenital hypothyroidism: ensures adequate thyroid hormone for growth and neurodevelopment in infants.
• Goiter associated with hypothyroidism: reduces gland enlargement by correcting hormone deficiency.

• Levothyroxine sodium is a synthetic form of the endogenous hormone thyroxine (T4). After oral absorption, it is converted to the active hormone triiodothyronine (T3) within peripheral tissues. T3 binds nuclear thyroid hormone receptors, modulating gene transcription that regulates basal metabolic rate, protein synthesis, and growth. By restoring circulating T4/T3 concentrations, the drug normalizes the hypothalamic‑pituitary‑thyroid feedback loop and re‑establishes physiological metabolic activity.

Common Side Effects

• Headache: mild to moderate discomfort often reported during therapy initiation.
• Insomnia: difficulty falling or staying asleep, especially if taken later in the day.
• Palpitations: sensation of rapid or irregular heartbeat.
• Heat intolerance: increased sweating and feeling overly warm.
• Tremor: fine shaking of hands or limbs.
• Weight loss: unintended reduction in body weight.

Serious or Rare Side Effects

• Cardiac arrhythmia: potentially life‑threatening irregular heart rhythms.
• Angina or myocardial infarction: chest pain or heart attack in susceptible patients.
• Osteoporosis: accelerated bone loss with long‑term overtreatment.
• Severe allergic reaction: rash, swelling, or anaphylaxis.
• Hyperthyroidism symptoms: severe anxiety, tremor, or fever indicating excess hormone levels.

Before initiating therapy, consider the following precautions:

• Dose adjustment: required in elderly patients, and those with renal or hepatic impairment.
• Cardiovascular disease: monitor for tachycardia, angina, or arrhythmias.
• Pregnancy: use only if clearly needed, after risk‑benefit assessment.
• Breastfeeding: assess infant exposure and maternal need.
• Thyroid function monitoring: check TSH and free T4 levels regularly to guide dosing.
• Store at ambient temperature: keep in the original container, protect from moisture and light.

Levothyroxine may interact with several medicines that affect its absorption or metabolism.

Major Interactions (Avoid)

• Antacids containing aluminum or magnesium: significantly reduce levothyroxine absorption.
• Calcium supplements or calcium‑containing foods: decrease oral bioavailability.
• Iron preparations: impair absorption when taken concurrently.
• Warfarin: levothyroxine may enhance anticoagulant effect, requiring INR monitoring.

Moderate Interactions (Monitor Closely)

• Beta‑blockers: may mask symptoms of excess thyroid hormone.
• Oral contraceptives or estrogen therapy: increase thyroid‑binding globulin, potentially altering dose requirements.
• Iodine‑containing contrast agents: can transiently affect thyroid function tests.