Levothyroxine
SYNTHROID
What it isThyroid-receptor agonist supplied as synthetic T4; standard replacement for hypothyroidism.
Why this oneIts long, stable action supports precise TSH-guided replacement; liothyronine cannot match that steadiness.
What limits itTake consistently on an empty stomach and separate calcium or iron. Recheck TSH after formulation, dose or interaction changes.
FDA-approved for
- hypothyroidism (~1955)
FDA label
Read the full label on DailyMed ↗
| FDA dosing | Population | Start | Target | Max |
|---|---|---|---|---|
| Primary/secondary/tertiary hypothyroidism | Adults | 1.6 mcg/kg QD (full replacement); some require lower | Titrate to euthyroid / normal TSH | Doses >200 mcg/day seldom required |
| Hypothyroidism | Adults at risk of AF / with cardiac disease or elderly | <1.6 mcg/kg QD (lower start) | Titrate slowly to euthyroid | |
| Congenital/acquired hypothyroidism | Pediatric (weight/age-based) | 10–15 mcg/kg QD (0–3 mo) down to 1.6 mcg/kg QD when growth complete | Titrate to euthyroid | |
| Pre-existing hypothyroidism in pregnancy | Pregnant | Pre-pregnancy dose, often increased 12.5–25 mcg/day | TSH within trimester-specific range | |
| TSH suppression in well-differentiated thyroid cancer | Adults & pediatric | Dose targeted to desired TSH suppression | Per cancer stage/status |
Titration Adults: adjust 12.5–25 mcg every 4–6 weeks (every 6–8 weeks if cardiac/elderly); pediatric: every 2 weeks.
Renal Undefined
Hepatic Undefined
Narrow therapeutic index; peak therapeutic effect may take 4–6 weeks.
Instructions
- Take once daily on an empty stomach, 30–60 minutes before breakfast.
- Administer at least 4 hours before or after drugs that impair absorption.
- Pediatric patients unable to swallow tablets: crush and suspend freshly in 5–10 mL water, give immediately by spoon/dropper; do not give in soybean-based formula.
- Stop biotin supplements >=2 days before assessing TSH/T4.
- Return to pre-pregnancy dose immediately after delivery.
Contraindications
- Uncorrected adrenal insufficiency
Cautions
- Overtreatment/undertreatment harms (narrow therapeutic index)
- Cardiac adverse reactions in elderly and cardiovascular disease, incl. atrial fibrillation — start lower dose
- Do not use oral thyroid hormone to treat myxedema coma
- Acute adrenal crisis if adrenal insufficiency — give glucocorticoids first
- May worsen glycemic control in diabetes
- Decreased bone mineral density with over-replacement (esp. postmenopausal women)
Adverse reactions
- Signs of hyperthyroidism from overdose: palpitations, tachycardia, arrhythmias
- Weight loss, increased appetite, heat intolerance, sweating
- Tremor, nervousness, anxiety, insomnia, headache
- Diarrhea, vomiting, abdominal cramps
- Decreased bone mineral density; menstrual irregularities
- Pediatric: pseudotumor cerebri, slipped capital femoral epiphysis, craniosynostosis
- Hypersensitivity to inactive ingredients (urticaria, rash, angioedema)
Pregnancy No increased rate of birth defects/miscarriage with euthyroid maintenance; do not discontinue; requirements often increase — monitor TSH and adjust.
Lactation Present in human milk; no adverse effects on breastfed infant reported.
⚠ BOXED WARNING
- Not for treatment of obesity or weight loss
Cost, est. cash$4–$80/month
Generic entry2002
Classes and tags
Clinical profile
Legacy pregnancy categoryA
Defining liabilityOver-replacement causes atrial fibrillation and bone loss; overdose produces prolonged thyrotoxicosis.
Mechanism of action: Levothyroxine ballicule
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Memory aids: mascots and interaction avatars
Levothyroxine (T4) contains 4 iodine (I) atoms
