What is the normal dosing for Synthroid (levothyroxine)?

Normal Dosing for Synthroid (Levothyroxine)

The normal starting dose of Synthroid (levothyroxine) for adults with hypothyroidism is 1.6 mcg/kg/day, with subsequent titration by 12.5 to 25 mcg increments every 4-6 weeks until the patient is euthyroid and TSH returns to normal range. 1

Adult Dosing Guidelines

• For adults with primary hypothyroidism without cardiac disease or advanced age, the full replacement dose is 1.6 mcg/kg/day 1
• Dosage titration should occur in 12.5 to 25 mcg increments every 4 to 6 weeks until the patient is euthyroid and TSH normalizes 1
• Dosages greater than 200 mcg/day are seldom required, and inadequate response to daily dosages greater than 300 mcg/day may indicate poor compliance, malabsorption, or drug interactions 1
• The peak therapeutic effect of a given dose may not be attained for 4 to 6 weeks 1

Special Populations

Elderly Patients

• For geriatric patients, a lower starting dose (less than 1.6 mcg/kg/day) is recommended 1
• Recent research from the Baltimore Longitudinal Study of Aging suggests older adults (≥65 years) typically require approximately 1.09 μg/kg or 1.35 μg/kg of ideal body weight, which is about one-third lower than recommendations for younger populations 2
• This reduced requirement is due to slower thyroid hormone metabolism with advancing age 3, 2

Patients with Cardiac Disease

• For adults at risk for atrial fibrillation or with underlying cardiac disease, a lower starting dose (less than 1.6 mcg/kg/day) is recommended 1
• Titration should be more gradual, with dosage adjustments every 6 to 8 weeks to avoid exacerbation of cardiac symptoms 1

Pediatric Dosing Guidelines

Levothyroxine dosing in children is based on body weight and age 1:

• 0 to 3 months: 10 to 15 mcg/kg/day 1
• 3 to 6 months: 8 to 10 mcg/kg/day 1
• 6 to 12 months: 6 to 8 mcg/kg/day 1
• 1 to 5 years: 5 to 6 mcg/kg/day 1
• 6 to 12 years: 4 to 5 mcg/kg/day 1

• 12 years but growth and puberty incomplete: 2 to 3 mcg/kg/day 1

• Growth and puberty complete: 1.6 mcg/kg/day 1

Monitoring and Dose Adjustment

• For primary hypothyroidism, monitor serum TSH to guide dosage adjustments 1
• For secondary or tertiary hypothyroidism, serum TSH is not reliable; use serum free-T4 levels to titrate dosing until the patient is clinically euthyroid and free-T4 is restored to the upper half of the normal range 1
• Assess adequacy of therapy through periodic laboratory tests and clinical evaluation 1
• Persistent clinical and laboratory evidence of hypothyroidism despite an apparently adequate replacement dose may indicate inadequate absorption, poor compliance, or drug interactions 1

Common Pitfalls and Considerations

• Administer levothyroxine at least 4 hours before or after drugs known to interfere with absorption 1
• Take on an empty stomach, 30-60 minutes before breakfast, to maximize absorption 4
• Certain foods can affect absorption if taken within one hour of medication 1
• About 25% of patients on levothyroxine are unintentionally maintained on doses sufficient to fully suppress TSH, highlighting the importance of regular monitoring 4
• Overtreatment with levothyroxine can lead to iatrogenic hyperthyroidism, increasing risk for osteoporosis, fractures, abnormal cardiac output, and ventricular hypertrophy 4
• The levothyroxine replacement dose varies with the cause of hypothyroidism - patients with atrophic thyroiditis typically require lower doses than those with Hashimoto's thyroiditis or post-radioiodine hypothyroidism, while patients with central hypothyroidism or thyroid cancer typically require higher doses 5