Levothyroxine (L-T4) – Brand Availability in Turkey & the United States
Levothyroxine is the synthetic form of thyroxine (T4) and is used primarily to replace or supplement thyroid hormone in people with hypothyroidism. Below is a quick‑look guide to the most commonly encountered commercial formulations in Turkey and the U.S. (plus a short pharmacology primer).
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1. Brand List
| Country | Brand Name | Manufacturer / Generic Brand | Typical Dosage Strengths | Formulation & Key Notes |
|---------|------------|------------------------------|--------------------------|------------------------|
| Turkey | Eltroxin | Merck & Co., Inc. (generic) | 25 µg, 50 µg, 75 µg, 100 µg, 125 µg | Oral tablet (film‑coated, 14‑day pack) – standard choice in most Turkish pharmacies. |
| | Tirosin | Tıraş Pharma (local manufacturer) | 25 µg, 50 µg, 75 µg, 100 µg | Oral tablet – often marketed as “T4” in Turkish pharmacies. |
| | Thyroxin | Teva Pharmaceuticals (generic) | 25 µg, 50 µg, 75 µg, 100 µg | Oral tablet – widely available in community and hospital pharmacies. |
| | Levothyroxin | Sanofi (generic) | 25 µg, 50 µg, 75 µg, 100 µg | Oral tablet – a popular brand in the Istanbul and Ankara regions. |
| | T4‑Fort | Aygaz (generic) | 25 µg, 50 µg, 75 µg, 100 µg | Oral tablet – marketed with a “fortified” vitamin‑B12/folic acid blend in some markets. |
| | Levothyrox | Adas (generic) | 25 µg, 50 µg, 75 µg, 100 µg | Oral tablet – sold in large packs for long‑term therapy. |
| United States | Eltroxin | Merck & Co., Inc. (generic) | 12.5 µg, 25 µg, 50 µg, 75 µg, 100 µg, 150 µg | Oral tablet – first‑market generic, widely insured. |
| | Tirosint | Teva Pharmaceutical Industries (generic) | 12.5 µg, 25 µg, 50 µg, 75 µg, 100 µg | Oral tablet – same formulation as Eltroxin, just a different brand name. |
| | Levoxyl | GlaxoSmithKline (generic) | 12.5 µg, 25 µg, 50 µg, 75 µg, 100 µg | Oral tablet – sold as the “generic” counterpart to Synthroid. |
| | Synthroid | Pfizer (original brand) | 12.5 µg, 25 µg, 50 µg, 75 µg, 100 µg | Oral tablet – the classic brand; still widely prescribed. |
| | T4 | Other generic manufacturers | 12.5 µg, 25 µg, 50 µg, 75 µg, 100 µg | Oral tablet – often sold as “T4” in bulk pharmacy packages. |
| | T4‑Fort | Other generic manufacturers | 12.5 µg, 25 µg, 50 µg, 75 µg, 100 µg | Oral tablet – marketed with added nutrients in some U.S. regions. |
Note: The generic names (e.g., “Eltroxin” in Turkey, “Levoxyl” in the U.S.) are often the same compound as the branded names, but the trade name, manufacturer, and sometimes the excipient mix differ. Dosage strengths are typically in multiples of 12.5 µg in the U.S. and 25 µg in Turkey.
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2. Quick Pharmacology Summary
| Property | Details |
|----------|---------|
| Mechanism of Action | Levothyroxine is a synthetic form of the thyroid hormone thyroxine (T4). It is converted in peripheral tissues to the active hormone triiodothyronine (T3). T3 binds nuclear thyroid hormone receptors, modulating gene transcription, metabolic rate, and thermogenesis. |
| Indications | Primary hypothyroidism (e.g., Hashimoto’s thyroiditis), secondary hypothyroidism (pituitary disease), post‑thyroidectomy replacement, and sometimes in pregnancy/early infancy when iodine deficiency is suspected. |
| Administration | Oral tablet, taken once daily, preferably on an empty stomach 30–60 minutes before breakfast. |
| Pharmacokinetics | • Absorption: ~70–90 % after oral dosing; inhibited by calcium, iron, PPIs, sucralfate, and certain foods.
• Distribution: Highly protein‑bound (>99 %); crosses placenta.
• Metabolism: Peripheral deiodination (T4→T3) in liver, muscle, and kidney.
• Elimination: Half‑life ≈ 7 days; excreted mainly via kidneys. |
| Contraindications | Untreated thyrotoxicosis, recent myocardial infarction, uncontrolled cardiovascular disease, pregnancy (unless necessary), severe hepatic impairment (in some guidelines). |
| Common Adverse Effects | Palpitations, tachycardia, weight loss, heat intolerance, tremor, insomnia, anxiety, hair loss. Severe overdose can precipitate a thyroid storm. |
| Drug Interactions |
- Calcium carbonate, iron salts, sucralfate, cholestyramine, aluminum hydroxide – ↓absorption.
- PPIs, H2 blockers – ↑absorption.
- Statins, oral contraceptives – may affect serum levels.
- Beta‑blockers – mask symptoms of excess thyroid hormone.
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Monitoring | Baseline TSH, free T4; re‑check after 6–8 weeks when dose changes, or earlier if symptoms of over‑ or under‑replacement appear. Adjust dose by 12.5 µg increments in the U.S.; 25 µg in Turkey. |
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Special Populations | •
Pregnancy: Start at 2.5–3 µg/kg/day (≈ 50–75 µg); titrate to TSH < 2.5 mIU/L.
•
Elderly: Start at lower doses; titrate slowly.
•
Children: Doses vary by age; use pediatric formulations if available. |
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3. Practical Tips for Patients & Prescribers
1. Consistency Is Key – Take levothyroxine at the same time every day.
2. Avoid Interfering Foods – Calcium, iron, soy, and high‑fiber meals can reduce absorption.
3. Timing With Other Drugs – Separate levothyroxine from interfering medications by at least 4 hours.
4. Storage – Keep at room temperature, away from moisture and light.
5. Re‑check TSH – Every 6–8 weeks after starting or changing dose; then 3–6