How do you calculate levothyroxine (Synthroid) dosing?

Levothyroxine (Synthroid) Dosing Guidelines

The recommended starting dose of levothyroxine for hypothyroidism is 1.6 mcg/kg/day for most adults, with dose adjustments of 12.5-25 mcg every 4-6 weeks based on TSH levels until euthyroidism is achieved. 1, 2

Initial Dosing Considerations

• For adults without cardiac disease or other significant comorbidities, the full replacement dose is 1.6 mcg/kg/day 1, 2
• For patients >70 years or with cardiac disease/multiple comorbidities, start with a lower dose of 25-50 mcg/day and titrate gradually to avoid cardiac complications 2, 1
• For children, dosing is weight and age-dependent, ranging from 10-15 mcg/kg/day for infants 0-3 months to 1.6 mcg/kg/day for adolescents with completed growth and puberty 1

Dose Titration Protocol

• Titrate dosage by 12.5-25 mcg increments every 4-6 weeks until the patient is euthyroid and TSH returns to normal range 1, 2
• For elderly patients or those with cardiac disease, use smaller increments (12.5 mcg) and titrate more slowly every 6-8 weeks 1, 3
• Monitor TSH every 6-8 weeks while titrating hormone replacement 2
• Once adequately treated, repeat testing every 6-12 months or if symptoms change 2

Special Populations

• Pregnant women: Increase levothyroxine dosage by 30% up to nine doses per week (take one extra dose twice weekly) as soon as pregnancy is confirmed 2, 4
• Monitor TSH every 4 weeks during pregnancy until stable dose is reached 1
• Return to pre-pregnancy dosage immediately after delivery 1
• For patients at risk of hyperactivity (particularly children): Start at one-fourth the recommended full replacement dosage and increase weekly by one-fourth until full dose is reached 1

Common Pitfalls to Avoid

• Approximately 25% of patients on levothyroxine are unintentionally maintained on doses sufficient to fully suppress TSH, highlighting the importance of regular monitoring 2
• Overtreatment with levothyroxine can lead to iatrogenic hyperthyroidism, increasing risk for osteoporosis, fractures, abnormal cardiac output, and ventricular hypertrophy 2
• Undertreatment risks include persistent hypothyroid symptoms, adverse effects on cardiovascular function, lipid metabolism, and quality of life 2
• Dosages greater than 200 mcg/day are seldom required; inadequate response to daily dosages greater than 300 mcg/day is rare and may indicate poor compliance, malabsorption, or drug interactions 1

Administration Guidelines

• Administer levothyroxine as a single daily dose, on an empty stomach, 30-60 minutes before breakfast with a full glass of water 1
• Take levothyroxine at least 4 hours before or after drugs known to interfere with absorption 1
• For patients who cannot swallow tablets, crush the tablet and suspend in a small amount of water (5-10 mL) for immediate administration 1

By following these evidence-based dosing guidelines and monitoring protocols, clinicians can effectively manage hypothyroidism while minimizing the risks of under or overtreatment.