Last reviewed 2026-04-20 · How we verify

Levoxyl (Levothyroxine Sodium)

Levothyroxine binds thyroid receptors in cell nuclei to activate gene transcription and protein synthesis.

Levothyroxine sodium (Levoxyl) is a synthetic L-thyroxine (T4) replacement therapy indicated for hypothyroidism and TSH suppression in thyroid cancer management. The drug has a long half-life of 6-7 days, high protein binding (99.96%), and absorption of 40-80% from the gastrointestinal tract, with relative bioavailability of approximately 93% compared to solution formulations. Key risks include uncorrected adrenal insufficiency as an absolute contraindication, and numerous drug interactions affecting absorption through binding or pH changes, requiring careful timing of administration and TSH monitoring. Levothyroxine remains the standard thyroid hormone replacement therapy with well-established efficacy when appropriately dosed and monitored.

At a glance

Mechanism of action

Thyroid hormones exert their physiologic actions through control of DNA transcription and protein synthesis. Triiodothyronine (T3) and L-thyroxine (T4) diffuse into the cell nucleus and bind to thyroid receptor proteins attached to DNA. This hormone nuclear receptor complex activates gene transcription and synthesis of messenger RNA and cytoplasmic proteins. The physiological actions of thyroid hormones are produced predominantly by T3, the majority of which (approximately 80%) is derived from T4 by deiodination in peripheral tissues.

Approved indications

• Primary hypothyroidism - replacement therapy
• Secondary hypothyroidism - replacement therapy
• Tertiary hypothyroidism - replacement therapy
• Congenital hypothyroidism - replacement therapy
• Acquired hypothyroidism - replacement therapy
• TSH suppression in thyrotropin-dependent well-differentiated thyroid cancer - adjunct to surgery
• TSH suppression in thyrotropin-dependent well-differentiated thyroid cancer - adjunct to radioiodine therapy
• Hypothyroidism in adult patients
• Hypothyroidism in pediatric patients
• Hypothyroidism in neonates

Boxed warnings

• WARNING: NOT FOR TREATMENT OF OBESITY OR FOR WEIGHT LOSS Thyroid hormones, including levothyroxine sodium, either alone or with other therapeutic agents, should not be used for the treatment of obesity or for weight loss. In euthyroid patients, doses within the range of daily hormonal requirements are ineffective for weight reduction. Larger doses may produce serious or even life-threatening manifestations of toxicity, particularly when given in association with sympathomimetic amines such as those used for their anorectic effects [see Adverse Reactions (6) , Drug Interactions (7.7) , and Overdosage (10) ]. WARNING: NOT FOR TREATMENT OF OBESITY OR FOR WEIGHT LOSS See full prescribing information for complete boxed warning. Thyroid hormones, including levothyroxine sodium, should not be used for the treatment of obesity or for weight loss. Doses beyond the range of daily hormonal requirements may produce serious or even life-threatening manifestations of toxicity (6 , 10) .

Common side effects

• Hypertension
• Tachycardia
• ANXIETY
• Atrial fibrillation
• FATIGUE
• Ventricular ectopy
• Ventricular tachycardia
• Misc/Other

Drug interactions

• Phosphate binders (calcium carbonate, ferrous sulfate, sevelamer, lanthanum)
• Orlistat
• Bile acid sequestrants (colesevelam, cholestyramine, colestipol)
• Ion exchange resins (Kayexalate)
• Proton pump inhibitors
• Sucralfate
• Antacids (aluminum & magnesium hydroxides, simethicone)

Key clinical trials

• Novel Approaches to the Treatment of Hypothyroidism (PHASE2, PHASE3)
• Comparison of Levothyroxine Formulations in the Treatment of Congenital Hypothyroidism
• T4/T3 Therapy in Hypothyroidism (PHASE2)
• SNUH Immune Checkpoint Inhibitor-induced Hypothyroidism Recovery Trial (NA)
• Thyroid Hormone for Treatment of Nonalcoholic Steatohepatitis in Veterans (PHASE2)
• Dosing of LT4 in Older Individuals (PHASE4)
• A Study Evaluation the Safety and Efficacy of Hormone Replacement Therapy With North Star Compared to Levothyroxine in Patients With Primary Hypothyroidism (PHASE2)
• Precision Medicine Approach for Early Dementia & Mild Cognitive Impairment (PHASE3)

Patents

Primary sources

Every claim on this page is sourced from regulatory or scientific primary sources. See our editorial policy for full methodology.

Competitive intelligence

For the full competitive landscape — auto-detected comparators, recent regulatory actions across the set, upcoming PDUFA, patent timeline, sponsor landscape:

• Levoxyl CI brief — competitive landscape report
• Levoxyl updates RSS · CI watch RSS
• Pfizer Inc. portfolio CI