Levothyroxine
Brand names: Synthroid, Levoxyl, Tirosint
Class: 🦋 Thyroid & Antithyroid
What examiners watch for is whether you can dose levothyroxine, time it correctly, and recheck TSH at the right interval. Levothyroxine replaces T4 in primary, central, and post-thyroidectomy hypothyroidism, and treats myxedema coma and thyroid cancer suppression. Healthy adult dose is approximately 1.6 mcg/kg/day; start lower (25–50 mcg) in older adults or known coronary disease and titrate slowly to avoid arrhythmia. Take on an empty stomach with water and separate calcium, iron, antacids, PPIs, and soy by four hours. Recheck TSH 6–8 weeks after any change — sooner adds noise. In pregnancy, demand rises about 30%; check TSH each trimester and target the trimester-specific range.
✅ Indications
Hypothyroidism (all causes), thyroid cancer suppression, myxedema coma.
⚙️ Mechanism of Action
Synthetic T4 — converted peripherally to active T3.
📏 Dosing
Adults: 1.6 mcg/kg/day. Elderly or CAD: start 12.5–25 mcg.
🚫 Contraindications
Uncorrected adrenal insufficiency, acute MI, thyrotoxicosis.
⚠️ Adverse Effects
Typically none at appropriate dose; overdose → symptoms of hyperthyroidism (palpitations, tremor, weight loss, insomnia).
🔬 Monitoring
TSH at 6–8 weeks after start or dose change; then annually once stable.
💎 Board Pearls
- 🕐 EMPTY STOMACH, 30–60 min before breakfast. Separate from calcium/iron/PPI/coffee by 4 hours.
- 👵 Elderly/CAD → start low (12.5–25 mcg) — avoid cardiac strain.
- 🤰 Pregnancy: needs ↑ ~30% by 4–6 weeks gestation; TSH target <2.5 first trimester.
- 📊 Recheck TSH at 6–8 WEEKS (long t½).
Practice Questions
A patient newly prescribed levothyroxine (Synthroid) 75 mcg daily for primary hypothyroidism asks how to take the medication. Which instruction is MOST accurate?
Related Drugs in This Class
- Methimazole — Tapazole
- Propylthiouracil — PTU
Sources
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