Last reviewed 2026-04-20 · How we verify

Synthroid (levothyroxine)

Synthetic thyroxine (T4) that replaces deficient thyroid hormone, restoring normal metabolism, growth, and development.

Levothyroxine (Synthroid) is the most prescribed medication in the United States, with over 100 million annual prescriptions. It replaces deficient thyroid hormone in hypothyroidism. Available generically from multiple manufacturers.

At a glance

Mechanism of action

Levothyroxine is synthetic T4, identical to the hormone produced by the thyroid gland. It is converted to the active T3 in peripheral tissues. It is consistently the most prescribed medication in the United States, with over 100 million prescriptions annually. Narrow therapeutic index requires careful dose titration based on TSH levels.

Approved indications

• Congenital hypothyroidism
• Diagnostic Test for Thyroid Dysfunction
• Goiter
• Hashimoto thyroiditis
• Hyperlipidemia
• Hypothyroidism
• Malignant tumor of thyroid gland
• Myxedema

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

• Palpitations
• Tachycardia
• Arrhythmias
• Headache
• Nervousness
• Anxiety
• Insomnia
• Tremors
• Diarrhea
• Heat intolerance
• Excessive sweating
• Fatigue

Serious adverse events

• Myocardial infarction
• Cardiac arrest
• Heart failure
• Angina
• Pseudotumor cerebri
• Slipped capital femoral epiphysis
• Craniosynostosis
• Seizures
• Angioedema
• Elevation in liver function tests

Key clinical trials

• Combined Therapy With L-Thyroxine and L-Triiodothyronine Compared to L-Thyroxine Alone in the Treatment of Primary Hypothyroidism (NA)
• Impact of Treating Thyroid Hormone Dysfunction During Pregnancy: A Randomized Controlled Trial of Universal Screening Versus Case Finding (NA)
• Effects of Discontinuation of Levothyroxine Treatment in Older Adults: a Self-controlled Study (N/A)
• Levothyroxine Dosing in Older Individuals (Phase 4)
• Single-Dose Fasting In Vivo Bioequivalence Study of Levothyroxine Sodium Tablets (200 mg; Mylan) to Synthroid Tablets (200 mg; Abbott) in Healthy Volunteers (Phase 1)
• Central Hypothyroidism,a New Laboratory Approach for Hormone Measurement (NA)
• NOMOTHETICOS: Nonlinear Modelling of Thyroid Hormones' Effect on Thyrotropin Incretion in Confirmed Open-loop Situation. A Correlation Study (N/A)
• PNOC044: Liothyronine (L-T3) in Combination With Bevacizumab, Irinotecan and Temozolomide (BIT) for Progressive or Relapsed Medulloblastoma (Cohort 1) or as Monotherapy for Medulloblastoma With Minima (Phase 1)

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:

• Synthroid CI brief — competitive landscape report
• Synthroid updates RSS · CI watch RSS
• Generic (originally multiple — AbbVie/Synthroid) portfolio CI