Levothyroxine Treatment and Dosage
Initial Dosing Strategy
For adults under 70 years without cardiac disease, start levothyroxine at the full replacement dose of approximately 1.6 mcg/kg/day (typically 75-100 mcg for women, 100-150 mcg for men), while elderly patients or those with cardiac disease should begin at 25-50 mcg/day with gradual titration. 1, 2, 3
Age-Based Dosing Approach
Younger Adults (<70 years, no cardiac disease):
- Full replacement dose: 1.6 mcg/kg/day 1, 2
- Typical range: 75-100 mcg/day for women, 100-150 mcg/day for men 3
- Can titrate by 25 mcg increments every 4-6 weeks 1, 2
Elderly Patients (>70 years) or Cardiac Disease:
- Start conservatively at 25-50 mcg/day 1, 2, 4
- Titrate by smaller increments (12.5-25 mcg) every 6-8 weeks 1, 2
- Elderly patients require approximately one-third less levothyroxine than younger adults (average 110-118 mcg/day vs 158 mcg/day) 5, 6
- This reflects the progressive decrease in thyroxine degradation rate with age 6
Obese Patients
For obese patients, use a conservative starting approach of 100-125 mcg daily, adjusting based on TSH results rather than strict weight-based dosing. 7
Monitoring and Dose Adjustment Protocol
Check TSH and free T4 every 6-8 weeks during dose titration, then every 6-12 months once stable. 1, 2
Titration Guidelines
- Adjust dose by 12.5-25 mcg increments based on current dose and patient factors 1, 7
- Wait 4-6 weeks between adjustments to reach steady state 1, 2
- Peak therapeutic effect may not occur for 4-6 weeks 2
- Target TSH: 0.5-4.5 mIU/L for most patients 1
Special Monitoring Situations
Patients with cardiac disease, atrial fibrillation, or serious medical conditions require more frequent monitoring within 2 weeks of dose adjustment. 1
Treatment Thresholds Based on TSH Levels
TSH >10 mIU/L:
- Initiate levothyroxine regardless of symptoms 1, 4
- Carries ~5% annual risk of progression to overt hypothyroidism 1
- May improve symptoms and lower LDL cholesterol 1
TSH 4.5-10 mIU/L (Subclinical Hypothyroidism):
- Routine treatment not recommended 1
- Consider treatment if: symptomatic, positive anti-TPO antibodies (4.3% vs 2.6% annual progression risk), pregnant, or planning pregnancy 1
- Otherwise, monitor TSH every 6-12 months 1
TSH <4.5 mIU/L:
- No treatment indicated 1
- Confirm elevated TSH with repeat testing after 3-6 weeks, as 30-60% normalize spontaneously 1, 4
Critical Safety Considerations
Risks of Overtreatment
Approximately 25% of patients on levothyroxine are unintentionally maintained on doses sufficient to fully suppress TSH, increasing risks for serious complications. 1
- Atrial fibrillation and cardiac arrhythmias (especially elderly) 1
- Osteoporosis and fractures (especially postmenopausal women) 1, 4
- Ventricular hypertrophy and abnormal cardiac output 1
- Increased cardiovascular mortality 1
When to Reduce Dose
If TSH <0.1 mIU/L: Decrease by 25-50 mcg 1 If TSH 0.1-0.45 mIU/L: Decrease by 12.5-25 mcg 1
Critical Pitfall to Avoid
Never initiate levothyroxine before ruling out adrenal insufficiency in patients with suspected central hypothyroidism, as this can precipitate adrenal crisis—always start corticosteroids first. 1
Pregnancy Considerations
Pregnant women with pre-existing hypothyroidism typically require 25-50% dose increases, with TSH monitoring as soon as pregnancy is confirmed and during each trimester. 1, 2
- Maintain TSH in trimester-specific reference range 2
- Subclinical hypothyroidism during pregnancy is associated with preeclampsia, low birth weight, and potential neurodevelopmental effects 1
Factors Affecting Dosage Requirements
Medications reducing absorption: Iron, calcium supplements—separate by 4 hours 4 Enzyme inducers: Reduce levothyroxine efficacy, may require dose increase 4 Weight changes: Require dose reassessment 8 Dosages >200 mcg/day are seldom required: Consider poor compliance, malabsorption, or drug interactions if inadequate response 2