What is the usual starting dose of levothyroxine?

Levothyroxine Starting Dose

For most adults with primary hypothyroidism, start levothyroxine at 1.6 mcg/kg/day of actual body weight, with lower starting doses reserved for elderly patients and those with cardiac disease. 1

Adult Dosing by Patient Population

Standard Adult Patients

• Full replacement dose: 1.6 mcg/kg/day based on actual body weight 1
• This represents the FDA-approved starting dose for adults diagnosed with hypothyroidism 1
• Research confirms this approach is safe and achieves euthyroidism faster than low-dose titration strategies 2

High-Risk Cardiac Populations

Use a lower starting dose (less than 1.6 mcg/kg/day) for: 1

• Adults at risk for atrial fibrillation
• Patients with underlying cardiac disease
• Geriatric patients

Titration in cardiac patients: Increase dosage every 6-8 weeks (rather than the standard 4-6 weeks) to avoid exacerbation of cardiac symptoms 1

Standard Adult Titration

• Adjust by 12.5 to 25 mcg increments every 4-6 weeks until euthyroid 1
• Monitor serum TSH to guide adjustments 1
• Peak therapeutic effect may not be attained for 4-6 weeks after each dose change 1

Pediatric Dosing

Dosing is weight-based and age-dependent: 1

• 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

Special pediatric considerations: 1

• Infants 0-3 months at risk for cardiac failure: Start lower and increase every 4-6 weeks
• Children at risk for hyperactivity: Start at one-fourth the full dose, increase weekly by one-fourth until reaching full dose

Pregnancy Considerations

For new-onset hypothyroidism in pregnancy: 1

• TSH ≥10 mIU/L: Start 1.6 mcg/kg/day
• TSH <10 mIU/L: Start 1.0 mcg/kg/day
• Monitor TSH every 4 weeks and adjust to maintain trimester-specific reference ranges 1

For pre-existing hypothyroidism: 1

• Increase pre-pregnancy dose by 12.5-25 mcg/day as needed
• Monitor TSH every 6-8 weeks during pregnancy 3
• Return to pre-pregnancy dose immediately postpartum 1

Important Clinical Considerations

Subclinical Hypothyroidism

• TSH 4.5-10 mIU/L: Routine treatment not recommended; monitor TSH every 6-12 months 3
• TSH >10 mIU/L: Levothyroxine therapy is reasonable given higher progression risk 3
• Pregnant women with any TSH elevation should be treated to restore TSH to reference range 3

Obesity Adjustments

• While the FDA recommends dosing by actual body weight 1, research suggests lean body mass may be more accurate in obese patients 4
• Obese patients may require lower doses per kg of actual body weight (approximately 1.4 mcg/kg in BMI ≥30) but similar doses when calculated by lean body mass 4

Common Pitfalls

• Inadequate response to >300 mcg/day suggests poor compliance, malabsorption, or drug interactions rather than need for higher doses 1
• Doses >200 mcg/day are seldom required 1
• For secondary/tertiary hypothyroidism, use free-T4 (not TSH) to guide therapy, targeting the upper half of normal range 1