Comparative Pharmacology
Head-to-head clinical analysis: LEVOXYL versus SYNTHROID.
Peer-Reviewed Evidence
Head-to-head clinical analysis: LEVOXYL versus SYNTHROID.
LEVOXYL vs SYNTHROID
Comparing the clinical profiles, pharmacokinetic behaviors, and safety indices of these two therapeutic agents.
Levothyroxine is a synthetic thyroid hormone (T4) that is deiodinated to triiodothyronine (T3) in peripheral tissues, binding to thyroid hormone receptors in the nucleus and increasing metabolic rate, protein synthesis, and oxygen consumption.
Synthetic levothyroxine is a replacement for endogenous thyroid hormone. It binds to thyroid hormone receptors (TRα and TRβ) in the nucleus, regulating gene transcription involved in metabolism, growth, and development.
Initial adult dose: 25-50 mcg orally once daily; titrate by 12.5-25 mcg every 6-8 weeks based on TSH. Maintenance dose: 50-200 mcg orally once daily.
Initial adult dose 1.6 mcg/kg orally once daily, adjusted by 12.5-25 mcg increments every 6-8 weeks based on TSH levels. Maintenance dose typically 100-125 mcg/day.
None Documented
None Documented
6-7 days in euthyroid patients; prolonged in hypothyroidism (9-10 days), shortened in hyperthyroidism (3-4 days); clinical steady-state after 6-8 weeks of consistent dosing.
Levothyroxine (T4) terminal elimination half-life: 6-7 days in euthyroid patients; shortened to 3-4 days in hyperthyroidism and prolonged to 9-10 days in hypothyroidism; clinical context: supports once-daily dosing with steady-state reached after 6-8 weeks.
Renal (30-50% as unchanged drug and conjugates); fecal (biliary, 20-40% as conjugates); total clearance approximates 1-2 L/day in euthyroid patients.
Renal: ~20-40% of T4 and T3 metabolites excreted in urine as glucuronide and sulfate conjugates; fecal: ~40-60% as unchanged drug and conjugates via biliary elimination; minor amounts in bile and feces as deiodinated products.
Category C
Category C
Thyroid Hormone
Thyroid Hormone