Levothyroxine Mercury Pharmaceuticals Ltd. (Levothyroxine Sodium)

Levothyroxine sodium is indicated for conditions involving insufficient thyroid hormone production.

• Hypothyroidism: restores normal metabolic rate and alleviates symptoms such as fatigue, weight gain, and cold intolerance.
• Subclinical hypothyroidism: normalizes elevated TSH levels and helps prevent progression to overt disease.
• Post‑thyroidectomy or radioactive iodine therapy: provides necessary hormone replacement to maintain metabolic homeostasis.

• Levothyroxine sodium 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, where it binds nuclear thyroid hormone receptors and modulates gene transcription that controls basal metabolic rate, protein synthesis, and growth.

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

Common Side Effects

• Headache: mild to moderate headache reported especially during dose adjustments.
• Insomnia: difficulty falling or staying asleep.
• Palpitations: sensation of rapid or irregular heartbeat.
• Weight loss: unintended loss of body weight.
• Heat intolerance: increased sensitivity to warm temperatures.
• Tremor: fine shaking of the hands.

Serious or Rare Side Effects

• Cardiac arrhythmia: potentially life‑threatening irregular heart rhythm.
• Angina pectoris: chest pain due to reduced coronary blood flow.
• Myocardial infarction: heart attack in susceptible patients.
• Severe hyperthyroidism: symptoms of excess thyroid hormone such as severe tachycardia.
• Osteoporosis: decreased bone mineral density with long‑term overtreatment.
• Anaphylaxis: rare allergic reaction with rash, swelling, and breathing difficulty.

When prescribing Levothyroxine sodium, clinicians should consider several safety measures to minimize risk.

• Dose titration in elderly: start with lower doses and adjust gradually.
• Monitor thyroid function: regular TSH and free T4 testing to ensure appropriate dosing.
• Cardiovascular disease caution: evaluate for angina or arrhythmias before dose increase.
• Adrenal insufficiency: ensure adequate glucocorticoid replacement before initiating therapy.
• Pregnancy and lactation: assess risk‑benefit and adjust dose based on thyroid function tests.
• Store at ambient temperature: keep below 25 °C, retain in original container, protect from moisture and light.

Levothyroxine sodium can interact with several medicines, affecting its absorption or hormonal activity.

Major Interactions (Avoid)

• Calcium or calcium carbonate supplements: bind levothyroxine and markedly reduce absorption.
• Iron salts (ferrous sulfate): significantly decrease levothyroxine bioavailability.
• Antacids containing aluminum or magnesium: impair absorption if taken concurrently.
• Sucralfate: forms complexes that prevent levothyroxine uptake.
• Cholestyramine or colestipol: bile‑acid sequestrants that lower levothyroxine levels.

Moderate Interactions (Monitor Closely)

• Estrogen therapy (oral contraceptives, hormone replacement): may increase levothyroxine requirement.
• Anticonvulsants (phenytoin, carbamazepine): enhance hepatic metabolism, reducing effectiveness.
• Rifampin: induces hepatic enzymes leading to lower levothyroxine concentrations.
• Amiodarone: alters peripheral conversion of T4 to T3, requiring dose adjustments.