Synthroid
A daily replacement hormone for an underactive thyroid.
Where Levothyroxine is approved
All regulators →Of the three regulators tracked here, the first to approve Levothyroxine was the US, on 21 Aug 2000. It is approved in 2 of the 3 regulators tracked here.
From the regulators' own registers: FDA Drugs@FDA, the EMA medicines list and Health Canada's Notice of Compliance database. Dates are the first listed approval of a product containing only this substance; combination products and nationally authorised EU medicines are not counted.
Levothyroxine patents and exclusivity
Expiry tracker →Protections still running for marketed products containing Levothyroxine, as listed by the regulators. Expired patents and exclusivities, and discontinued products, are not shown.
- 15 listed patents; the last listed expires 26 Sep 2044 (US 12564565).
Reproduced from the FDA Orange Book and Purple Book (checked 5 Oct 2026) and the European Commission's Union Register of medicinal products (CC BY 4.0; the EU expiry date is calculated exactly as the Commission's register displays it). Listed patents and dates are as submitted to or published by the regulator and may not reflect litigation, invalidation, settlements, lapsed fees or later extensions. This is not legal advice or a freedom-to-operate opinion, and it does not say when a generic or biosimilar will launch.
What health systems spend on Levothyroxine
All medicines →From NHSBSA Prescription Cost Analysis (Open Government Licence v3.0) and CMS State Drug Utilization Data. Both are gross amounts: neither system publishes its discounts or rebates per drug. Products combining several substances are not counted here.
How Levothyroxine works
All drug targets →- Thyroid hormone receptor agonist AgonistTarget: Thyroid hormone receptor✓ In ChEMBL and the Guide to Pharmacology
- Down syndromePhase 3
- Intraventricular hemorrhagePhase 3
- Patent ductus arteriosusPhase 3
- ProteinuriaPhase 3
- ShockPhase 3
- AbortionPhase 3
- Bronchopulmonary dysplasiaPhase 3
- Chronic kidney diseasePhase 3
- DeafnessPhase 3
- Differentiated thyroid carcinomaPhase 3
Mechanisms and diseases from ChEMBL via the Open Targets Platform (release 26.09), ChEMBL record CHEMBL1624, CC BY-SA 3.0. Targets checked against the IUPHAR/BPS Guide to PHARMACOLOGY (2026.3), ligand 2635, CC BY-SA 4.0. Trial phases are not approvals: approved uses are shown from the regulators' own records where we hold them.
Patient leaflets for Levothyroxine
The package leaflet is the official guide for patients that comes with every medicine. These links go to the regulators that publish it.
- IEIreland: 30 products on the HPRA listEach product record links to the HPRA, which publishes the leaflet and SmPC.
- UKUnited Kingdom: Search MHRA ProductsPatient information leaflets and SmPCs for UK-licensed products.
- USUnited States: Search DailyMedFDA-approved labels, including patient and medication guides.
Leaflets differ between brands and countries; always read the one that comes with your own medicine. This is regulatory information, not medical advice.
Reference identifiers for Levothyroxine
- ATC code
- H03AA01
- ChEMBL
- CHEMBL1624
- DrugBank
- DB00451
- PubChem CID
- 25201348
- CAS number
- 51-48-9
- FDA UNII
- Q51BO43MG4
- Wikidata
- Q773449
- Wikipedia
- Levothyroxine
Codes that identify this medicine in other databases, from Wikidata (CC0), matched by its ChEMBL ID. The ATC code is the WHO classification of what the medicine is used for.
What Synthroid is used for
Hypothyroidism SYNTHROID is indicated in adult and pediatric patients, including neonates, as a replacement therapy in primary (thyroidal), secondary (pituitary), and tertiary (hypothalamic) congenital or acquired hypothyroidism. Pituitary Thyrotropin (Thyroid -Stimulating Hormone, TSH) Suppression SYNTHROID is indicated in adult and pediatric patients, including neonates, as an adjunct to surgery and radioiodine therapy in the management of thyrotropin-dependent well-differentiated thyroid cancer. Limitations of Use SYNTHROID is not indicated for suppression of benign thyroid nodules and nontoxic diffuse goiter in iodine-sufficient patients as there are no clinical benefits and overtreatment with SYNTHROID may induce hyperthyroidism [see Warnings and Precautions ( 5.1 )]. SYNTHROID is not indicated for treatment of hypothyroidism during the recovery phase of subacute thyroiditis. SYNTH…
How it works
12.1 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.
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How this page is built
The facts on this page are pulled directly from official U.S. FDA datasets — they are not written from memory. Each field below names the dataset it came from, so you can check it yourself.
- FDA Drug Label (openFDA) ↗ Indications, mechanism of action and pharmacologic class
- Drugs@FDA ↗ First FDA approval date and the company holding the application
- FDA Drug Shortage Database ↗ Current US supply / shortage status
Plain-English summaries and drug-class explainers are written and reviewed by Sreepriya Prasannan (MSc Digital Transformation of Life Sciences (Innopharma Education / Griffith College); MSc & BSc Botany). Data is retrieved automatically from the sources above and cross-checked with AI-assisted verification (Anthropic's Claude) — brand and generic names are matched against the exact FDA product record so that a combination product or a different formulation cannot be mistaken for the drug on this page. An editor reviews the result before publication. We describe this in full in our editorial standards and corrections policy. The FDA data on this page was last retrieved on 4 Oct 2026. How every register is built: methodology · fixes we have made: corrections log.