What is the typical starting dose of levothyroxine (T4) for adults?

Starting Dose of Levothyroxine for Adults

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 those with cardiac disease, start at 25-50 mcg/day and titrate gradually. 1, 2

Dosing Algorithm Based on Patient Characteristics

Younger Adults Without Cardiac Disease (<70 years, no heart disease)

• Start at full replacement dose of 1.6 mcg/kg/day based on ideal body weight 1, 2
• This approach is safe and reaches euthyroidism faster than low-dose titration (13 vs 1 patient euthyroid at 4 weeks in clinical trials) 3
• Titrate by 12.5-25 mcg increments every 4-6 weeks based on TSH levels until euthyroid 1, 2

Elderly Patients (>70 years) or Those With Cardiac Disease

• Start at 25-50 mcg/day to avoid cardiac complications 1, 2
• Elderly patients require approximately one-third less levothyroxine than younger adults (mean dose 1.09 mcg/kg vs 1.6 mcg/kg) 4, 5
• Titrate more slowly, every 6-8 weeks, to minimize risk of exacerbating cardiac symptoms 1, 2
• For obese elderly patients, calculate dose using ideal body weight (1.35 mcg/kg IBW) rather than actual body weight 4

Patients at Risk for Atrial Fibrillation

• Start below 1.6 mcg/kg/day and titrate cautiously every 6-8 weeks 2
• Monitor closely for cardiac arrhythmias, as TSH suppression significantly increases atrial fibrillation risk 1

Monitoring and Titration Protocol

Initial Monitoring Phase

• Check TSH and free T4 every 6-8 weeks during dose titration 1
• Peak therapeutic effect of a given dose may not be attained for 4-6 weeks 2
• Target TSH within reference range (0.5-4.5 mIU/L) with normal free T4 1

Maintenance Monitoring

• Once adequately treated, repeat TSH testing every 6-12 months 1
• Monitor more frequently if symptoms change or new medications are started 1

Special Populations Requiring Modified Dosing

Pregnant Patients

• Increase levothyroxine dose by 25-50% as soon as pregnancy is confirmed 1
• Monitor TSH and free T4 each trimester, maintaining TSH in trimester-specific reference range 2

Patients With Central Hypothyroidism

• Do not use TSH to guide dosing—titrate based on free T4 levels 2
• Target free T4 in the upper half of normal range 2
• Critical: Rule out adrenal insufficiency first and start corticosteroids before levothyroxine to prevent adrenal crisis 1

Common Pitfalls to Avoid

Dosing Errors

• Avoid starting elderly or cardiac patients at full replacement dose—this risks cardiac decompensation, angina, or arrhythmias 1, 5
• Do not adjust doses more frequently than every 4-6 weeks before reaching steady state 1
• Approximately 25% of patients are unintentionally maintained on excessive doses that fully suppress TSH, increasing risks for atrial fibrillation, osteoporosis, and fractures 1

Monitoring Errors

• Do not rely on a single TSH measurement—confirm elevated TSH with repeat testing after 3-6 weeks, as 30-60% normalize spontaneously 1, 6
• For patients with pituitary disease, do not use TSH alone—always check free T4 as TSH may be inappropriately normal despite hypothyroidism 1

Evidence Quality Considerations

The full-dose approach for younger adults is supported by high-quality prospective randomized controlled trial data showing safety and faster achievement of euthyroidism without cardiac events 3. The conservative approach for elderly patients is based on pharmacokinetic studies demonstrating reduced thyroid hormone metabolism with age 4, 5. The FDA-approved dosing guidelines align with these evidence-based recommendations 2.