What should the Levothyroxine dosage be?

Levothyroxine Dosing

For adults under 70 years without cardiac disease, start with 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 and titrate slowly. 1

Initial Dosing Strategy

Standard Adult Dosing (Age <70, No Cardiac Disease)

• Begin with full replacement dose of 1.6 mcg/kg/day based on ideal body weight 1, 2
• This typically translates to 75-100 mcg/day for women and 100-150 mcg/day for men 3
• The full replacement approach allows faster achievement of euthyroid state in patients without contraindications 1

High-Risk Populations Requiring Lower Starting Doses

• Patients >70 years old: Start at 25-50 mcg/day 4, 1
• Patients with atrial fibrillation or cardiac disease: Start at 25-50 mcg/day 4, 1
• Patients with multiple comorbidities: Start at 25-50 mcg/day 4
• Lower starting doses prevent exacerbation of cardiac symptoms and arrhythmias 1

Pediatric Dosing (Weight-Based)

• 0-3 months: 10-15 mcg/kg/day 1
• 3-6 months: 8-10 mcg/kg/day 1
• 6-12 months: 6-8 mcg/kg/day 1
• 1-5 years: 5-6 mcg/kg/day 1
• 6-12 years: 4-5 mcg/kg/day 1
• >12 years (growth incomplete): 2-3 mcg/kg/day 1
• Growth complete: 1.6 mcg/kg/day 1

Dose Titration Protocol

Standard Titration Schedule

• Increase dose by 12.5-25 mcg increments every 4-6 weeks for standard adults 1, 5
• For elderly or cardiac patients, titrate every 6-8 weeks with smaller increments (12.5 mcg) 4, 5, 1
• Larger increments (25 mcg) are appropriate for younger patients without cardiac disease 4
• The peak therapeutic effect may not be attained for 4-6 weeks after dose adjustment 1

Monitoring During Titration

• Check TSH and free T4 every 6-8 weeks while adjusting doses 4, 5, 1
• Target TSH within reference range (0.5-4.5 mIU/L) with normal free T4 4, 5
• Free T4 helps interpret ongoing abnormal TSH levels, as TSH may take longer to normalize 4, 5

Special Clinical Situations

Pregnancy Considerations

• Pregnant women with pre-existing hypothyroidism require 25-50% dose increases 4
• Check TSH and free T4 as soon as pregnancy is confirmed 1
• Monitor at minimum during each trimester, as requirements typically increase by 30% or more by 4-6 weeks' gestation 5, 1
• Maintain TSH in trimester-specific reference ranges 6, 1

TSH Suppression for Thyroid Cancer

• Low-risk patients with excellent response: Target TSH 0.5-2 mIU/L 4
• Intermediate/high-risk with biochemical incomplete response: Target TSH 0.1-0.5 mIU/L 4
• Structural incomplete response: Target TSH <0.1 mIU/L 4
• Consultation with endocrinology is recommended for cancer patients requiring TSH suppression 4

Subclinical Hypothyroidism Treatment Thresholds

• TSH >10 mIU/L: Initiate levothyroxine regardless of symptoms 4
• This level carries approximately 5% annual risk of progression to overt hypothyroidism 4
• TSH 4.5-10 mIU/L: Consider treatment if symptomatic, pregnant, or positive TPO antibodies 4
• For patients already on levothyroxine with TSH 4.5-10 mIU/L, dose adjustment is reasonable 4

Long-Term Monitoring

Maintenance Phase

• Once stable, check TSH every 6-12 months 4, 5
• Repeat testing sooner if symptoms change 4
• Approximately 25% of patients are inadvertently maintained on excessive doses that suppress TSH, increasing risks for atrial fibrillation, osteoporosis, and cardiac complications 4, 5

Dose Reduction for Overtreatment

• TSH <0.1 mIU/L: Decrease dose by 25-50 mcg 4
• TSH 0.1-0.45 mIU/L: Decrease dose by 12.5-25 mcg 4
• Recheck in 6-8 weeks after adjustment, or within 2 weeks for patients with atrial fibrillation or serious cardiac disease 4

Critical Pitfalls to Avoid

• Never start levothyroxine before ruling out adrenal insufficiency in suspected central hypothyroidism - this can precipitate adrenal crisis 4
• Do not treat based on single elevated TSH - 30-60% normalize on repeat testing 4
• Avoid adjusting doses more frequently than every 4-6 weeks - steady state is not reached sooner 4
• Do not administer within 4 hours of medications that interfere with absorption (calcium, iron, proton pump inhibitors) 1
• Overtreatment occurs in 14-21% of patients and significantly increases cardiovascular and bone risks 4