What is the recommended dosage of thyroxine (T4) for a patient with hypothyroidism?

Levothyroxine Dosing for Hypothyroidism

Initial Dosing Strategy

For healthy adults under 50 years without cardiac disease, start levothyroxine at the full replacement dose of approximately 1.6 mcg/kg/day (typically 100-125 mcg/day for a 70 kg adult). 1 This approach is safe, reaches therapeutic targets faster, and is more cost-effective than gradual titration in this population. 2

Age and Cardiac Risk-Based Dosing

For patients over 50 years OR those under 50 with underlying cardiac disease:

• Start at 25-50 mcg/day 3, 1
• Increase gradually at 6-8 week intervals 3, 1
• Use 12.5-25 mcg increments for dose adjustments 4, 3

For elderly patients with cardiac disease (>70 years or known coronary disease):

• Start at 12.5-25 mcg/day 3, 1
• Titrate at 4-6 week intervals 1
• Use smaller 12.5 mcg increments to minimize cardiovascular risk 4, 3
• This cautious approach prevents exacerbation of coronary artery disease and arrhythmias 3

For patients with severe hypothyroidism:

• Start at 12.5-25 mcg/day 1
• Increase by 25 mcg every 2-4 weeks 1
• Monitor clinical and laboratory parameters until TSH normalizes 1

Target TSH and Monitoring

Target TSH: 0.5-4.5 mIU/L with normal free T4 levels 4

• Monitor TSH every 6-8 weeks during dose titration 4, 3
• Once stable, monitor TSH every 6-12 months 4
• Levothyroxine requires 4-6 weeks to reach steady state 1

Special Populations

Pregnancy

• Increase weekly levothyroxine dose by 30% immediately upon pregnancy confirmation (take one extra dose twice per week) 5
• Monitor TSH monthly during pregnancy 4
• Target TSH <2.5 mIU/L in first trimester 4

Subclinical Hypothyroidism

• If treated, a lower dose of approximately 1 mcg/kg/day may be adequate to normalize TSH 1
• Treatment is recommended for TSH >10 mIU/L regardless of symptoms 4
• For TSH 4.5-10 mIU/L, treatment decisions should be individualized based on symptoms, pregnancy status, or positive anti-TPO antibodies 4

Thyroid Cancer (TSH Suppression)

• Low-risk patients: Target TSH 0.5-2 mIU/L 4
• Intermediate-to-high risk: Target TSH 0.1-0.5 mIU/L 4
• Structural incomplete response: Target TSH <0.1 mIU/L 4
• This typically requires doses >2 mcg/kg/day 1

Critical Safety Considerations

Always rule out adrenal insufficiency before starting levothyroxine, especially in suspected central hypothyroidism. 4, 3 Starting thyroid hormone before corticosteroids can precipitate life-threatening adrenal crisis. 4, 3 If adrenal insufficiency is present, start corticosteroids several days before initiating levothyroxine. 3

Administration Guidelines

• Take levothyroxine in the morning on an empty stomach 1
• Wait at least 30-60 minutes before eating 1
• Separate from iron, calcium supplements, or antacids by at least 4 hours 1

Common Pitfalls to Avoid

Overtreatment (TSH <0.1 mIU/L) significantly increases risks: 4, 3

• Atrial fibrillation (especially in elderly) 4, 3
• Osteoporosis and fractures 4, 3
• Cardiovascular complications 4, 3
• Approximately 25% of patients are unintentionally maintained on excessive doses 4

Undertreatment (persistent TSH >10 mIU/L) leads to: 4, 3

• Persistent hypothyroid symptoms 4, 3
• Adverse cardiovascular effects 4, 3
• Abnormal lipid metabolism 4, 3
• Reduced quality of life 4, 3

Do not adjust doses more frequently than every 6-8 weeks before reaching steady state. 4

Dose Adjustment Protocol

• Adjust in 12.5-25 mcg increments based on TSH response 3, 1
• For elderly or cardiac patients, use smaller 12.5 mcg increments 4, 3
• If TSH becomes suppressed (<0.1-0.45 mIU/L), reduce dose by 12.5-25 mcg 4
• If TSH <0.1 mIU/L, reduce dose by 25-50 mcg immediately 4