What is the recommended starting dose of levothyroxine (T4) for an adult with primary hypothyroidism?

Levothyroxine Starting Dose for Adult Primary Hypothyroidism

For adults under 70 years without cardiac disease, start levothyroxine at the full replacement dose of 1.6 mcg/kg/day; for adults over 70 years or with cardiac disease, start at 25-50 mcg/day and titrate gradually. 1, 2, 3

Age and Cardiac Risk-Based Dosing Algorithm

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

• Start at full replacement dose of 1.6 mcg/kg/day to achieve rapid normalization of thyroid function 1, 2, 3
• This approach is safe and more cost-effective than gradual titration in cardiac asymptomatic patients 4
• Most patients in this category require 100-150 mcg/day (women) or 125-175 mcg/day (men) 5
• Euthyroidism is typically achieved within 4-8 weeks with full-dose initiation versus 20-24 weeks with low-dose titration 4

Elderly or Cardiac Risk Patients (>70 years OR cardiac disease)

• Start at 25-50 mcg/day to avoid exacerbating cardiac symptoms 1, 2, 3
• Titrate by 12.5-25 mcg increments every 6-8 weeks based on TSH response 1, 2
• Elderly patients require lower maintenance doses due to decreased thyroxine degradation rate with age (average 118 mcg/day in patients >65 years versus 158 mcg/day in younger adults) 6
• Patients with underlying coronary disease risk cardiac decompensation even with therapeutic levothyroxine doses 1

Monitoring and Titration Protocol

• Recheck TSH and free T4 after 6-8 weeks following initiation or any dose adjustment 1, 2, 3
• Target TSH within reference range (0.5-4.5 mIU/L) with normal free T4 1
• Adjust dose by 12.5-25 mcg increments based on TSH response 1, 2
• Peak therapeutic effect may not be attained for 4-6 weeks after dose adjustment 3
• Once stable, monitor TSH every 6-12 months 1

Special Populations Requiring Modified Approach

Pregnant Patients

• Increase weekly levothyroxine dosage by 30% (take one extra dose twice per week) as soon as pregnancy is confirmed 7
• Monitor TSH and free T4 each trimester, maintaining TSH in trimester-specific reference range 3
• Levothyroxine requirements increase 25-50% above pre-pregnancy doses 1

Patients with Suspected Adrenal Insufficiency

• Always start corticosteroids at least 1 week before initiating levothyroxine to prevent life-threatening adrenal crisis 1
• This is critical in patients with autoimmune hypothyroidism, central hypothyroidism, or hypophysitis 1

Critical Pitfalls to Avoid

• Never start full-dose levothyroxine in elderly patients or those with cardiac disease without gradual titration, as this can unmask or worsen cardiac ischemia 1, 2
• Avoid overtreatment—approximately 25% of patients are unintentionally maintained on doses sufficient to fully suppress TSH, increasing risks for atrial fibrillation, osteoporosis, and cardiac complications 1
• Do not treat based on a single elevated TSH value, as 30-60% normalize on repeat testing 1
• Confirm diagnosis with repeat TSH and free T4 after 3-6 weeks before initiating therapy 1

Dosage Considerations Based on Pretreatment TSH

• Higher pretreatment TSH levels correlate with higher optimal maintenance doses 8
• Most patients (65%) ultimately require 100-150 mcg/day maintenance dose 8
• Dosages greater than 200 mcg/day are seldom required; inadequate response to >300 mcg/day suggests poor compliance, malabsorption, or drug interactions 3