Levothyroxine Starting Dose for Adults with Hypothyroidism
The recommended starting dose of levothyroxine for adults with hypothyroidism is 1.6 mcg/kg/day for patients under 70 years without cardiac disease, while older patients or those with cardiac disease should start at a lower dose of 25-50 mcg/day. 1, 2
Dosing Based on Patient Characteristics
Standard Adult Dosing
- Full replacement dose is 1.6 mcg/kg/day for adults diagnosed with hypothyroidism who are under 70 years without cardiac disease 1, 2
- Titrate dosage by 12.5 to 25 mcg increments every 4-6 weeks until the patient is euthyroid 2
- Most patients require between 100-150 mcg/day 3
Special Populations
- For patients over 70 years or with cardiac disease/risk for atrial fibrillation: start with 25-50 mcg/day 1, 2
- For elderly patients: Consider a lower dose of approximately 1.09 μg/kg or 1.35 μg/kg based on ideal body weight (about one-third lower than standard adult dosing) 4
- Titrate more slowly (every 6-8 weeks) in elderly patients or those with cardiac disease 1, 2
Dosing Based on TSH Levels
Overt Hypothyroidism
- For patients with overt hypothyroidism (elevated TSH and low free T4), replacement therapy is clearly indicated 1, 5
- Higher pretreatment TSH levels correlate with higher ultimate levothyroxine requirements 3
Subclinical Hypothyroidism
- For TSH >10 mIU/L with normal free T4: initiate levothyroxine therapy regardless of symptoms 1
- For TSH between 4.5-10 mIU/L with normal free T4: treatment decisions should be individualized based on symptoms, presence of TPO antibodies, and other risk factors 1, 5
Monitoring and Dose Adjustment
- Check TSH every 6-8 weeks while titrating the dose 1
- Target TSH should be within the reference range (0.5-4.5 mIU/L) 1
- Once stable, monitor TSH every 6-12 months 1
- Free T4 can help interpret abnormal TSH levels during therapy 1
Common Pitfalls to Avoid
- Overtreatment risks include iatrogenic hyperthyroidism, which increases risk for atrial fibrillation, osteoporosis, and fractures 1, 5
- About 25% of patients on levothyroxine are unintentionally maintained on doses sufficient to fully suppress TSH 1
- Undertreatment risks include persistent hypothyroid symptoms and adverse effects on cardiovascular function 1
- Failure to recognize transient hypothyroidism may lead to unnecessary lifelong treatment 1
- Adjusting doses too frequently before reaching steady state (should wait 4-6 weeks between adjustments) 1
Administration Considerations
- Administer levothyroxine on an empty stomach, at least 30-60 minutes before food 2
- Certain medications (iron, calcium) and foods can reduce levothyroxine absorption 2, 6
- Dosages greater than 200 mcg/day are seldom required; inadequate response to >300 mcg/day may indicate poor compliance, malabsorption, or drug interactions 2
Special Clinical Scenarios
- For women planning pregnancy: more aggressive normalization of TSH is warranted 1, 6
- For patients with both adrenal insufficiency and hypothyroidism: steroids should always be started prior to thyroid hormone to avoid an adrenal crisis 1
- For patients with thyroid cancer requiring TSH suppression: target TSH levels may be intentionally lower, requiring consultation with an endocrinologist 1