Overview
Levothyroxine sodium is a synthetic thyroid hormone (T4) used as replacement therapy for hypothyroidism and as adjunctive treatment for thyroid cancer. It requires careful dose titration based on clinical response and laboratory parameters due to its narrow therapeutic index. Proper administration timing relative to food and other medications is critical for optimal absorption.
Mechanism of Action
Levothyroxine sodium is a synthetic form of thyroxine (T4) that replaces endogenous thyroid hormone. It is converted to the active form triiodothyronine (T3) in peripheral tissues, restoring normal thyroid hormone levels and metabolic processes in patients with thyroid hormone deficiency.
Indications
- Replacement therapy for primary, secondary, and tertiary hypothyroidism in adults and pediatric patients
- Adjunct to surgery and radioiodine therapy in management of thyrotropin-dependent well-differentiated thyroid cancer
Common Doses
- 25 mcg
- 50 mcg
- 75 mcg
- 88 mcg
- 100 mcg
- 112 mcg
- 125 mcg
- 137 mcg
- 150 mcg
- 175 mcg
- 200 mcg
- 300 mcg
Dosage
Administer as single daily dose on empty stomach, 30-60 minutes before breakfast with water. Starting dose varies by age, weight, and cardiac status: adults typically 1.6 mcg/kg/day, pediatric patients 10-15 mcg/kg/day for neonates decreasing with age. Titrate based on TSH or free-T4 levels every 4-8 weeks until euthyroid.
Contraindications
- Uncorrected adrenal insufficiency
Side Effects
- Symptoms of hyperthyroidism: palpitations, tachycardia, arrhythmias, angina
- Nervousness, anxiety, insomnia, tremors
- Headache, fatigue, heat intolerance, excessive sweating
- Increased appetite, weight loss
- Diarrhea, vomiting, abdominal cramps
- Hair loss, rash, flushing
- Menstrual irregularities
- Pseudotumor cerebri and slipped capital femoral epiphysis in pediatric patients
- Hypersensitivity reactions to inactive ingredients
Interactions
- Drugs that decrease absorption: calcium carbonate, iron supplements, proton pump inhibitors, sucralfate, bile acid sequestrants - administer 4 hours apart
- Drugs affecting thyroid hormone metabolism: phenobarbital, rifampin, carbamazepine, phenytoin
- Drugs decreasing T4 to T3 conversion: amiodarone, propranolol, glucocorticoids
- Increased anticoagulant effect with warfarin - monitor INR
- Decreased digitalis effect
- Increased catecholamine effects with antidepressants and sympathomimetics
- Biotin supplements interfere with thyroid function tests - discontinue 2 days before testing
Counseling Points
- Take on empty stomach 30-60 minutes before breakfast with full glass of water
- Separate from calcium, iron, antacids, and other interfering medications by at least 4 hours
- Consistency in timing and administration is crucial for stable thyroid levels
- Report symptoms of over- or under-treatment: palpitations, weight changes, fatigue, heat/cold intolerance
- Do not stop during pregnancy - dose adjustments are common and necessary
- Regular blood tests are needed to monitor therapy effectiveness