Euthyrox
Levothyroxine sodium
Generic Name
Levothyroxine sodium
Mechanism
- Glucocorticoid‑like hormone that replaces or supplements endogenous thyroid hormone.
- Interacts with nuclear thyroid hormone receptors (TR‑α and TR‑β) in target tissues, regulating transcription of genes involved in basal metabolic rate.
- Converted in peripheral tissues to the active moiety triiodothyronine (T3) by deiodinases (DIO1/DIO2), providing endocrine feedback.
- Restores normal sympathetic tone, protein synthesis, and carbohydrate metabolism.
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Pharmacokinetics
| Property | Detail |
| Absorption | Oral – ~70 % CE recovered; peak plasma 4–6 h (fasted). Highly variable. |
| Distribution | Protein‑bound > 99 % (α‑1‑acid glycoprotein). Volume of distribution ≈ 30 L. |
| Metabolism | Primarily deiodination to T3 (∼30 %) and T2/iodide. Minor conjugation. |
| Elimination | Renal excretion (≈ 30 % free T4). Half‑life ≈ 7 days (steady‑state). |
| Drug interactions | Chelators (Ca, Al, iron) ↓ absorption; PPIs ↑; estrogen ↑ clearance; liothyronine ↓. |
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Indications
- Primary hypothyroidism (all etiologies).
- Hashimoto’s thyroiditis and post‑thyroidectomy replacement.
- Congenital hypothyroidism (initiation < 2 weeks).
- Low‑iodine diet correction/pre‑operative prep.
- Antithyroid drug withdrawal replacement.
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Contraindications
- Untreated thyrotoxicosis or acute thyroid storm.
- Uncontrolled cardiac disease (angina, arrhythmias).
- Pregnancy: dose adjustment; avoid sudden withdrawal.
- Type II diabetes: risk of glucose intolerance.
Warnings:
• Rapid titration can precipitate ischemic heart disease.
• Elderly susceptible to arrhythmias, osteoporosis.
• Monitor thyroid‑sparing hormones closely during pregnancy/ lactation.
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Dosing
- Adult initial dose: 25–50 µg/day (start ≤ 50 µg in > 65 yr, renal impairment, cardiac disease).
- Titration: 25 µg increments every 4–6 weeks; target TSH: 0.5–4.0 mIU/L.
- Maximum: 200 µg/day (rare).
- Administration: Take orally with a full glass of water, 10–15 min before breakfast on an empty stomach.
- Form: Extended‑release tablets (Euthyrox®) or standard; same dosing schedule.
Adverse Effects
| Symptom | Common? | Severity |
| Palpitations, tachycardia | ✓ | Mild–moderate |
| Anxiety, irritability | ✓ | Mild |
| Weight loss | ✓ | Mild |
| Insomnia | ✓ | Mild |
| Musculoskeletal pain | ✓ | Mild |
| Reflex syncope | ✓ | Moderate |
| Serious: atrial fibrillation, myocardial infarction, osteoporosis, sudden death (rare, dose‑linked) |
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Monitoring
| Test | Frequency | Rationale |
| TSH & free T4 | Baseline, 4–6 weeks after dose change, then every 6–12 months | Dose titration |
| ECG & vitals | Baseline in elderly, cardiac disease; repeat if symptomatic | Cardiac safety |
| Serum calcium | 6–12 months; consider PM bone density in long‑term therapy | Osteoporosis risk |
| Pregnancy labs | TSH every 1–2 weeks (first trimester) | Fetal neurodevelopment |
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Clinical Pearls
- Early‑life initiation: In neonates, start within 48 h of birth; monitor neurodevelopment.
- Rate‑of‑titration guideline: Under 1 mIU/L change in TSH per 6 weeks to avoid ischemic events.
- CKD considerations: Slower metabolism; start at ½ dose, titrate over several months.
- Micronutrient interaction: Magnesium and zinc supplements *do not* impair absorption; avoid high‑dose multivitamins within 4 h.
- Travel & jet lag: Maintain regular dosing; avoid late‑night doses to prevent insomnia.
- Pregnancy: Fetomaternal TSH ≤ 2.5 mIU/L optimal; dose adjustment may be needed every 1–2 weeks.
Take‑home point: *Euthyrox* effectively restores euthyroid state when titrated cautiously, but vigilance for cardiovascular and bone health is essential because even modest over‑replacement can lead to significant morbidity.