Levothyroxine Dose (LT4)
Estimates the levothyroxine dose for thyroid hormone replacement.
Formula:
Adult standard: approximately 1.6 mcg/kg/day. In older adults or patients with cardiac disease, treatment is usually started cautiously at 12.5-25 mcg/day. In children, age-based dosing ranges are used.
Interpretation:
Initial LT4 dose estimate; adjustment is based on clinical response and TSH/FT4 monitoring.
Patient instructions
- Take levothyroxine with water on an empty stomach, ideally first thing in the morning.
- Wait 30-60 minutes before eating or taking other medications.
- Separate calcium, iron, and multivitamin supplements from levothyroxine by at least 4 hours.
- If evening dosing is preferred, take levothyroxine 3-4 hours after the last meal or snack.
- Requirements far above the weight-based estimate, especially above 200-300 mcg/day, may suggest malabsorption or nonadherence and should prompt further evaluation.
Management
- Prescribe the closest available oral dose, using tablet combinations, splitting, or alternate-day dosing when needed.
- If intravenous therapy is required, use about 30% lower dosing than oral therapy because intravenous bioavailability is higher.
- For follow-up, monitor TSH and clinical response every 6-8 weeks and adjust by 12.5-25 mcg when needed.
- Consult endocrinology for complex, refractory, or uncertain thyroid disease management.
Pediatric dosing ranges
- <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
- 12-18 years: 2-3 mcg/kg/day