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

Levothyroxine Starting Dose for Adults with Hypothyroidism

The recommended starting daily dosage of levothyroxine for adults with primary hypothyroidism is 1.6 mcg/kg/day for most patients without cardiac disease or risk factors. 1, 2, 3

Initial Dosing Based on Patient Characteristics

• For adults without cardiac disease or significant comorbidities, a full replacement dose of 1.6 mcg/kg/day is safe and effective 1, 4
• For adults >70 years or with cardiac disease/multiple comorbidities, start with a lower dose of 25-50 mcg/day and titrate gradually to avoid exacerbating cardiac symptoms 1, 2
• For patients at risk of atrial fibrillation or with underlying cardiac disease, use a lower starting dose and titrate more slowly (every 6-8 weeks) 2

Dose Considerations for Special Populations

• For pregnant women with pre-existing hypothyroidism, monitor TSH every 6-8 weeks and adjust dosage to maintain TSH within trimester-specific reference ranges 5
• For patients with secondary or tertiary hypothyroidism, use serum free-T4 levels (not TSH) to guide therapy, aiming for the upper half of the normal range 2
• For patients with TSH >10 mIU/L, treatment is recommended regardless of symptoms due to higher risk of progression to overt hypothyroidism 1

Monitoring and Titration

• After initiating therapy, monitor TSH every 6-8 weeks while titrating the dose 1
• Adjust dosage by 12.5-25 mcg increments every 4-6 weeks until the patient is euthyroid 1, 2
• The peak therapeutic effect may not be attained for 4-6 weeks after a dose adjustment 2
• Once adequately treated, repeat testing every 6-12 months or if symptoms change 1

Administration Guidelines

• Administer levothyroxine on an empty stomach, at least 30 minutes before breakfast for optimal absorption 6
• Take separately from other medications that may impair absorption 2, 6
• For patients who cannot swallow tablets, crush the tablet and suspend in a small amount of water for immediate administration 2

Common Pitfalls to Avoid

• Undertreatment risks persistent hypothyroid symptoms, adverse effects on cardiovascular function, lipid metabolism, and quality of life 1
• Overtreatment can lead to iatrogenic hyperthyroidism, increasing risk for osteoporosis, fractures, abnormal cardiac output, and ventricular hypertrophy 1
• About 25% of patients on levothyroxine are unintentionally maintained on doses sufficient to fully suppress TSH, highlighting the importance of regular monitoring 1
• Absorption may be impaired by medications that increase gastric pH or by certain foods like soybean-based products 2, 6

Practical Dosing Algorithm

1. Calculate weight-based dose (1.6 mcg/kg/day) for most adults 2, 3
2. Adjust initial dose based on age and cardiac status:
   • Adults <70 years without cardiac disease: Full replacement dose 1, 4
   • Adults >70 years or with cardiac disease: 25-50 mcg/day 1, 2
3. Monitor TSH every 6-8 weeks and adjust by 12.5-25 mcg increments 1, 2
4. Target normal TSH within reference range (0.4-4.0 mIU/L) 3
5. Once stable, monitor every 6-12 months 1