Recommended Levothyroxine Dosing for Thyroid Replacement Therapy
The recommended starting dose of levothyroxine for thyroid replacement therapy in adults with hypothyroidism is 1.6 mcg/kg/day based on ideal body weight, with lower starting doses (25-50 mcg/day) for elderly patients or those with cardiac disease. 1
Initial Dosing Based on Patient Characteristics
- For most adults with primary hypothyroidism, the full replacement dose is 1.6 mcg/kg/day based on ideal body weight 1
- For elderly patients (>60 years) or those with underlying cardiac disease, start with a lower dose of 25-50 mcg/day to avoid exacerbation of cardiac symptoms 1, 2
- For patients at risk of atrial fibrillation, use a lower starting dose (less than 1.6 mcg/kg/day) and titrate more slowly 1
Pediatric Dosing
- Dosing in pediatric patients varies by age and weight 1:
- 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 but growth incomplete: 2-3 mcg/kg/day
- Growth and puberty complete: 1.6 mcg/kg/day
Dose Titration and Monitoring
- For adults with primary hypothyroidism, titrate dosage by 12.5-25 mcg increments every 4-6 weeks until the patient is euthyroid and serum TSH returns to normal 1
- For elderly patients or those with cardiac disease, titrate more slowly every 6-8 weeks 1
- Monitor TSH every 6-8 weeks while titrating hormone replacement 2
- Once stabilized, repeat testing every 6-12 months or as indicated for a change in symptoms 2
- The peak therapeutic effect of a given dose may not be attained for 4-6 weeks 1
Special Populations
Pregnancy
- For pregnant patients with pre-existing hypothyroidism, increase levothyroxine dosage by 12.5-25 mcg/day as soon as pregnancy is confirmed 1
- Monitor TSH every 4 weeks until a stable dose is reached and serum TSH is within normal trimester-specific range 1
- The requirement for levothyroxine in treated hypothyroid women frequently increases during pregnancy 2
- For newly diagnosed hypothyroidism during pregnancy with TSH ≥10 IU/L, start at 1.6 mcg/kg/day 1
Subclinical Hypothyroidism
- For subclinical hypothyroidism with TSH >10 mIU/L, levothyroxine therapy is reasonable 3, 2
- For subclinical hypothyroidism with TSH between 4.5-10 mIU/L, routine treatment is not recommended unless the patient is symptomatic 3, 2
Administration Considerations
- Levothyroxine should be taken on an empty stomach, 30-60 minutes before breakfast or 3-4 hours after the evening meal 1
- Certain foods and medications can affect absorption, so consistent administration timing is important 1
- For patients who cannot swallow tablets, the tablet can be crushed and suspended in water for immediate administration 1
Common Pitfalls and Caveats
- Overtreatment can lead to subclinical hyperthyroidism, which may occur in 14-21% of individuals treated with levothyroxine 3, 2
- An inadequate response to daily dosages greater than 300 mcg/day is rare and may indicate poor compliance, malabsorption, or drug interactions 1
- Dosages greater than 200 mcg/day are seldom required 1
- Development of a low TSH on therapy suggests overtreatment or recovery of thyroid function, and dose should be reduced with close follow-up 2
- Adding triiodothyronine (T3) is not recommended, even in patients with persistent symptoms and normal TSH levels 4
By following these evidence-based dosing guidelines and monitoring protocols, clinicians can effectively manage hypothyroidism while minimizing the risks of under or overtreatment.