Levothyroxine Dosing for Primary Hypothyroidism
For adults under 50-70 years without cardiac disease, start levothyroxine at the full replacement dose of 1.6 mcg/kg/day; for older adults or those with cardiac disease, start at 25-50 mcg/day and titrate gradually. 1, 2, 3
Initial Dosing Strategy
Young, Healthy Adults (<50-70 years, no cardiac disease)
- Start at full replacement dose of 1.6 mcg/kg/day 1, 3, 4
- This approach is safe in cardiac asymptomatic patients and reaches euthyroidism faster than low-dose titration 5
- Most patients require 100-150 mcg/day, with a median dose of 125 mcg/day 6
- Dosages greater than 200 mcg/day are seldom required 3
Older Adults (>50-70 years) or Cardiac Disease
- Start at 25-50 mcg/day 1, 2, 3, 7
- This cautious approach prevents exacerbation of coronary artery disease and arrhythmias 2, 7
- Elderly patients are at higher risk for atrial fibrillation and cardiac complications with rapid titration 1, 2
Dose Titration Protocol
Adjustment Increments
- Increase by 12.5-25 mcg every 4-6 weeks based on TSH response 1, 3, 5
- For elderly or cardiac patients, use smaller 12.5 mcg increments 1, 2
- For younger patients without cardiac disease, 25 mcg increments are appropriate 1
Monitoring Schedule
- Recheck TSH every 6-8 weeks during dose titration 1, 2
- Wait at least 4-6 weeks between adjustments, as peak therapeutic effect takes this long to manifest 3, 7
- Once stable, monitor TSH every 6-12 months 1
Target TSH Levels
- Aim for TSH 0.4-4.5 mIU/L (or 0.5-4.0 mIU/L per some guidelines) 1, 4
- Free T4 should be in the normal range 1
- For secondary/tertiary hypothyroidism, target free T4 in the upper half of normal range instead of using TSH 3
Critical Safety Considerations
Adrenal Insufficiency
- Always start corticosteroids several days before thyroid hormone in patients with suspected adrenal insufficiency or hypophysitis 8, 2
- Starting levothyroxine before corticosteroids can precipitate adrenal crisis 8, 1
Cardiac Monitoring
- For patients at risk of atrial fibrillation or with underlying cardiac disease, titrate more slowly over 6-8 weeks 3
- No cardiac events occurred even with full-dose initiation in cardiac asymptomatic patients in prospective trials 5
Common Pitfalls to Avoid
Overtreatment Risks
- Suppressed TSH (<0.1 mIU/L) increases risk for atrial fibrillation, osteoporosis, fractures, and cardiac complications 1, 2, 7
- Approximately 25% of patients are inadvertently maintained on excessive doses 1
- Even slight overdose carries significant fracture risk, especially in elderly and postmenopausal women 1, 7
Undertreatment Risks
- TSH persistently >10 mIU/L indicates insufficient replacement 1, 2
- Persistent hypothyroid symptoms, cardiovascular dysfunction, abnormal lipid metabolism, and reduced quality of life result from inadequate dosing 1, 2
Drug Interactions
- Iron and calcium supplements reduce levothyroxine absorption 7
- Enzyme inducers reduce efficacy 7
- Take levothyroxine on an empty stomach 7
- Avoid soybean-based formulas which decrease absorption 3
Special Populations
Pregnancy
- Increase dose by 25-50% immediately upon pregnancy confirmation 1
- Pre-pregnancy levothyroxine requirements typically increase during pregnancy 1
- Monitor TSH every 4 weeks until stable, then each trimester 3
- Return to pre-pregnancy dose immediately postpartum 3