Starting Dose for Levothyroxine in a 74 kg Adult
For a 74 kg adult without cardiac disease or significant comorbidities, start levothyroxine at approximately 118 mcg daily (1.6 mcg/kg/day), which can be rounded to 112-125 mcg based on available tablet strengths. 1
Dosing Algorithm Based on Patient Characteristics
For Adults Under 60 Years Without Cardiac Disease
- Full replacement dose: 1.6 mcg/kg/day 2, 1, 3
- For this 74 kg patient: 74 kg × 1.6 mcg/kg = 118 mcg daily 1
- Round to nearest available tablet strength: 112 mcg or 125 mcg 1
- This full dose approach is safe and reaches euthyroidism faster than gradual titration 4
For Adults Over 60 Years or With Cardiac Disease
- Start with 25-50 mcg daily 2, 1, 5, 3
- Titrate by 12.5-25 mcg increments every 6-8 weeks 2, 1
- Lower starting doses prevent unmasking cardiac ischemia or precipitating arrhythmias 2, 3
For Obese Patients
- Use ideal body weight rather than actual body weight for dose calculation 6, 7
- Start conservatively at 100-125 mcg daily for obese patients 7
- Adjust based on TSH response after 6-8 weeks 7
Critical Monitoring and Adjustment
Initial Monitoring
- Recheck TSH and free T4 in 6-8 weeks after starting therapy 2, 1
- Peak therapeutic effect may not occur for 4-6 weeks 1
- Adjust dose by 12.5-25 mcg increments based on TSH results 2, 1
Target TSH Range
- Goal TSH: 0.5-4.5 mIU/L with normal free T4 2, 1
- Continue adjustments every 6-8 weeks until target achieved 2, 1
- Once stable, monitor TSH every 6-12 months 2
Administration Instructions
Timing and Absorption
- Take on empty stomach, 30-60 minutes before breakfast 2, 8, 3
- Separate from other medications by at least 4 hours, especially iron, calcium, and antacids 2, 8
- Absorption impaired by medications that increase gastric pH 8
Common Pitfalls to Avoid
Dosing Errors
- Never start elderly or cardiac patients at full replacement dose - this can precipitate myocardial infarction or fatal arrhythmias 2, 3
- Avoid adjusting doses more frequently than every 6-8 weeks before steady state is reached 2, 1
- Don't use actual body weight in obese patients - use ideal body weight 6, 7
Safety Considerations
- Rule out adrenal insufficiency before starting levothyroxine in suspected central hypothyroidism - thyroid hormone can precipitate adrenal crisis 2, 3
- Approximately 25% of patients are unintentionally overtreated with TSH suppression, increasing risks for atrial fibrillation and osteoporosis 2, 3
Monitoring Mistakes
- Don't rely on TSH alone in central hypothyroidism - use free T4 to guide dosing 1
- Avoid treating based on single elevated TSH value - 30-60% normalize on repeat testing 2, 5
Special Populations
Pregnant Women
- Increase pre-pregnancy dose by 25-50% immediately upon pregnancy confirmation 2, 3
- Target TSH <2.5 mIU/L in first trimester 2, 3
- Monitor TSH every 4 weeks until stable, then each trimester 2, 3