Synthroid (Levothyroxine) Dosing for Hypothyroidism
The recommended starting dose of levothyroxine for adult patients with primary hypothyroidism is 1.6 mcg/kg/day, with dose adjustments of 12.5 to 25 mcg every 4-6 weeks until the patient is euthyroid and TSH returns to normal. 1
Adult Dosing Guidelines
Primary Hypothyroidism
- Starting dose: 1.6 mcg/kg/day
- Titration: Increase by 12.5-25 mcg increments every 4-6 weeks
- Goal: Clinical euthyroidism with normal TSH
- Monitoring: TSH and free T4 levels
Special Populations
Adults with cardiac risk factors or underlying cardiac disease:
- Start with lower dose (less than 1.6 mcg/kg/day)
- Titrate more slowly (every 6-8 weeks)
- Reduces risk of exacerbating cardiac symptoms 1
Geriatric patients:
- Start with lower dose (less than 1.6 mcg/kg/day)
- Titrate more cautiously to avoid cardiovascular complications 1
Secondary or tertiary hypothyroidism:
- Monitor free T4 instead of TSH
- Target free T4 in the upper half of normal range 1
Pediatric Dosing Guidelines
Pediatric dosing is weight-based and age-dependent:
| Age | Starting Daily Dosage |
|---|---|
| 0-3 months | 10-15 mcg/kg/day |
| 3-6 months | 8-10 mcg/kg/day |
| 6-12 months | 6-8 mcg/kg/day |
| 1-5 years | 5-6 mcg/kg/day |
| 6-12 years | 4-5 mcg/kg/day |
| >12 years (growth incomplete) | 2-3 mcg/kg/day |
| Growth complete | 1.6 mcg/kg/day |
For pediatric patients at risk for hyperactivity, start at one-fourth the recommended dose and increase weekly by one-fourth until reaching full replacement dose 1.
Pregnancy Considerations
For pregnant patients with pre-existing hypothyroidism:
- Increase weekly dosage by 30% (take one extra dose twice weekly)
- Monitor TSH every 4 weeks until stable
- Maintain TSH within trimester-specific reference ranges
- Return to pre-pregnancy dose immediately after delivery 2, 1
Administration Guidelines
- Take levothyroxine as a single daily dose
- Take on an empty stomach, 30-60 minutes before breakfast
- Take with a full glass of water
- Avoid medications that interfere with absorption (calcium, iron supplements, antacids) for at least 4 hours 2, 1
Monitoring Therapy
- Initial follow-up: Check TSH and free T4 after 4-6 weeks of starting therapy
- Once stable: Monitor TSH at least annually
- Peak therapeutic effect may take 4-6 weeks to achieve 1
- Consider treatment failure if TSH remains abnormal despite doses >300 mcg/day (may indicate poor compliance, malabsorption, or drug interactions) 1
Important Considerations
- Dosages greater than 200 mcg/day are seldom required
- Inadequate response to doses >300 mcg/day may indicate poor compliance, malabsorption, or drug interactions 1
- For patients with adrenal insufficiency, corticosteroid replacement must be initiated before thyroid hormone replacement to avoid precipitating adrenal crisis 3, 2
- For thyroid cancer patients, TSH suppression therapy may require higher doses of levothyroxine 2
Remember that levothyroxine dosing must be individualized based on clinical response and laboratory parameters, with the ultimate goal of achieving clinical euthyroidism and normalization of TSH levels.