What is the starting dose of levothyroxine (T4) for an adult patient?

Starting Dose for Levothyroxine

For adults under 70 years without cardiac disease, start levothyroxine at the full replacement dose of 1.6 mcg/kg/day (typically 75-100 mcg for women, 100-150 mcg for men), while elderly patients or those with cardiac disease should start at 25-50 mcg/day with gradual titration. 1, 2

Age and Cardiac Risk-Based Dosing Algorithm

Healthy Adults (<70 years, no cardiac disease):

• Start at full replacement dose: 1.6 mcg/kg/day 1, 2, 3
• Typical doses: 75-100 mcg/day for women, 100-150 mcg/day for men 4
• This approach is safe and reaches euthyroidism faster than low-dose titration (13 vs 1 patient euthyroid at 4 weeks in one trial) 3
• Titrate by 12.5-25 mcg increments every 4-6 weeks based on TSH 2

Elderly Patients (>70 years) or Those with Cardiac Disease:

• Start at 25-50 mcg/day 1, 2, 5
• Elderly patients require approximately one-third less levothyroxine than younger adults 6
• Titrate more slowly, every 6-8 weeks, to avoid exacerbating cardiac symptoms 1, 2
• Even therapeutic doses can unmask or worsen cardiac ischemia in patients with underlying coronary disease 1

Patients at Risk for Atrial Fibrillation:

• Use lower starting dose (<1.6 mcg/kg/day) 2
• Titrate cautiously every 6-8 weeks 2
• Even slight overdose increases risk of atrial fibrillation 5-fold in patients ≥45 years 1

Pediatric Dosing

Age-based weight-adjusted dosing: 2

• 0-3 months: 10-15 mcg/kg/day
• 3-6 months: 8-10 mcg/kg/day
• 6-12 months: 6-8 mcg/kg/day
• 1-5 years: 5-6 mcg/kg/day
• 6-12 years: 4-5 mcg/kg/day

• 12 years (growth incomplete): 2-3 mcg/kg/day

• Growth complete: 1.6 mcg/kg/day

Critical Safety Considerations

Before initiating levothyroxine, always rule out adrenal insufficiency 1

• Starting thyroid hormone before corticosteroids can precipitate life-threatening adrenal crisis 1
• In suspected central hypothyroidism or hypophysitis, start physiologic dose steroids 1 week prior to levothyroxine 1

Cardiac monitoring is essential: 1, 5

• Elderly patients with coronary disease are at increased risk of cardiac decompensation 1
• No cardiac events occurred with full-dose initiation in cardiac asymptomatic patients in prospective trials 3
• However, caution remains warranted in symptomatic cardiac disease 2

Monitoring and Titration Timeline

Initial monitoring: 1, 2

• Recheck TSH and free T4 every 6-8 weeks during dose titration 1
• Peak therapeutic effect may not be attained for 4-6 weeks 2
• Target TSH: 0.5-4.5 mIU/L with normal free T4 1

Long-term monitoring: 1

• Once stable, monitor TSH every 6-12 months 1
• More frequent monitoring needed if symptoms change 1

Common Pitfalls to Avoid

Overtreatment occurs in 14-21% of treated patients 1

• Approximately 25% of patients are unintentionally maintained on doses sufficient to fully suppress TSH 1
• Overtreatment increases risk for atrial fibrillation, osteoporosis, fractures, and cardiac complications 1, 5

Undertreatment risks: 1

• Persistent hypothyroid symptoms
• Adverse cardiovascular effects
• Abnormal lipid metabolism
• Reduced quality of life

Drug and food interactions: 2

• Iron and calcium reduce gastrointestinal absorption 5
• Enzyme inducers reduce efficacy 5
• Administer on empty stomach 5
• Avoid soybean-based formulas in infants 2

Special Populations

Pregnant patients: 7, 2

• Pre-existing hypothyroidism often requires 25-50% dose increase during pregnancy 1
• Monitor TSH every 6-8 weeks during pregnancy 7
• Maintain TSH within trimester-specific reference ranges 2

Patients on immunotherapy: 1

• Thyroid dysfunction occurs in 6-20% on anti-PD-1/PD-L1 therapy 1
• Consider treatment even for subclinical hypothyroidism if fatigue present 1
• Continue immunotherapy in most cases 1