Levothyroxine Dosage for Adults
Initial Dosing Strategy
For adults under 70 years without cardiac disease, start with the full replacement dose of 1.6 mcg/kg/day (typically 100-150 mcg/day for most patients), as this approach safely achieves euthyroidism faster without increasing cardiac risk. 1, 2
- Most women require 100-150 mcg/day, while most men require 100-150 mcg/day, with a median dose of 125 mcg/day across all patients 3, 4
- The full replacement dose can be initiated immediately in younger patients without cardiac comorbidities 1, 2
Age and Cardiac Risk-Based Dosing
For patients over 70 years or those with cardiac disease/multiple comorbidities, start with a lower dose of 25-50 mcg/day and titrate more slowly every 6-8 weeks to avoid cardiac decompensation. 5, 1, 2
- Elderly patients require approximately one-third less levothyroxine than younger patients, with mean replacement doses around 110 mcg/day 6
- Lower starting doses minimize risk of exacerbating angina, arrhythmias, or precipitating cardiac events 5, 2
Dose Titration Protocol
Adjust levothyroxine by 12.5-25 mcg increments every 4-6 weeks based on TSH and free T4 levels until the patient is euthyroid. 1, 2
- For younger patients without cardiac disease, use 25 mcg increments 5, 1
- For elderly or cardiac patients, use smaller 12.5 mcg increments to avoid cardiac complications 5, 1
- Larger adjustments risk overtreatment and should be avoided 5, 1
- The peak therapeutic effect of a given dose may not be attained for 4-6 weeks 2
Monitoring Guidelines
Check TSH every 6-8 weeks during dose titration, then every 6-12 months once stable. 5, 1, 2
- For primary hypothyroidism, titrate until TSH normalizes to 0.5-4.5 mIU/L and the patient is clinically euthyroid 5, 1, 2
- Free T4 helps interpret ongoing abnormal TSH during therapy, as TSH may lag behind normalization 5, 1
- For secondary or tertiary hypothyroidism, TSH is unreliable—use serum free-T4 level and titrate until it reaches the upper half of the normal range 2
Special Population Considerations
For pregnant patients, increase the levothyroxine dose by 25-50% above pre-pregnancy requirements as soon as pregnancy is confirmed. 5, 1
- Inadequate treatment increases risk of preeclampsia, low birth weight, and neurodevelopmental effects in offspring 5
- Measure TSH and free T4 as soon as pregnancy is confirmed and during each trimester 2
For patients with TSH >10 mIU/L, initiate treatment regardless of symptoms, as this threshold carries approximately 5% annual risk of progression to overt hypothyroidism. 5, 1
Administration Timing
Administer levothyroxine on an empty stomach, at least 30-60 minutes before breakfast, to maximize absorption. 2, 7
- Taking levothyroxine before dinner instead of before breakfast results in significantly reduced therapeutic efficacy, with TSH increasing by 1.47 µIU/mL 7
- Administer at least 4 hours before or after drugs that interfere with absorption 2
Critical Pitfalls to Avoid
Never start levothyroxine before ruling out adrenal insufficiency in patients with suspected central hypothyroidism, as this can precipitate life-threatening adrenal crisis. 5, 1
- Approximately 25% of patients are unintentionally overtreated with TSH suppression, increasing risks for atrial fibrillation, osteoporosis, fractures, and cardiac complications 5, 1
- Prolonged TSH suppression (<0.1 mIU/L) increases risk for atrial fibrillation (especially in elderly), osteoporosis, and cardiovascular mortality 5, 1
- If TSH becomes suppressed, reduce dose by 12.5-25 mcg immediately 5, 1
Treatment Goals
Target TSH of 0.5-4.5 mIU/L for primary hypothyroidism; TSH suppression (<0.1 mIU/L) indicates overtreatment requiring immediate dose reduction. 5, 1