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Estimates the approximate full-replacement levothyroxine dose for adults using ~1.6 µg/kg/day (FDA levothyroxine labeling). It is NOT a universal prescribing dose calculator: the actual starting dose depends on age, duration and severity of hypothyroidism, cardiovascular status, pregnancy status, residual thyroid function, and clinical/laboratory findings.
Clinical decision-support tool for qualified healthcare professionals. Results are not a diagnosis and should not replace professional clinical judgment.
This calculator estimates only the approximate full-replacement levothyroxine dose (≈1.6 µg/kg/day) described in current FDA levothyroxine labeling. It is NOT a universal prescribing dose: the appropriate starting dose depends on age, duration and severity of hypothyroidism, cardiovascular status, pregnancy status, residual thyroid function, and clinical/laboratory findings. Elderly patients and patients with cardiac disease or atrial fibrillation risk generally require a lower starting dose (the FDA label cites 12.5–25 µg/day) and slower titration (typically every 6–8 weeks); standard adults are usually titrated in 12.5–25 µg increments every 4–6 weeks. Dose adequacy must be confirmed by clinical status and serum TSH (and/or free T4) rather than by calculator output alone, and usual doses seldom exceed 200 µg/day. Uncorrected adrenal insufficiency is a contraindication. Pregnancy-related dose management and thyroid cancer suppression dosing require separate clinical management and should not be guided by this calculator.
Specialty
EndocrinologyCategory
EndocrinologyDifficulty
Basic
Estimated Time
30 seconds
Version
1.0
Last Updated
2026-08-06
Formula
Keywords
Normal Range
Approximate full replacement dose ≈ 1.6 µg/kg/day (e.g. ~100–125 µg/day for a 70 kg adult). Elderly patients and patients with cardiac disease generally require lower starting doses, and the full replacement dose may be less than 1 µg/kg/day in the elderly. Titration is guided by clinical status, TSH, and/or free T4.
| Classification | Range |
|---|---|
Lower starting dose often appropriate elderly, cardiovascular disease, cardiac risk (treat cautiously) | <1.1 µg/kg/day |
Moderate dose dose range seen during titration | 1–1.6 µg/kg/day |
Full replacement dose average full replacement dose for otherwise appropriate adults per FDA labeling | ≈1.6 µg/kg/day |
Estimates a weight-based levothyroxine dose (≈1.6 µg/kg/day) using the calculator's implemented equation, illustrating the weight-based reference point used when starting levothyroxine replacement.
Estimated full-replacement levothyroxine dose = 1.6 micrograms/kg/day x body weight (kg). This is an approximation, not a universal starting dose.
Body weight in kilograms, multiplied by approximately 1.6 micrograms/kg/day.
The calculator reports the weight-based estimate (1.6 µg/kg/day) as the 'full replacement dose.' The calculator's only input is body weight — age, cardiac risk, and pregnancy are not inputs and are not reflected in the number. Actual starting doses are individualized; many patients, especially older adults or those with cardiac disease, are started at much lower doses (commonly 25–50 µg/day) and titrated using TSH.
A weight-based estimate (≈1.6 µg/kg/day) provides a reference point for full levothyroxine replacement, but real-world starting doses are individualized — particularly in older adults and patients with cardiac disease — and are titrated to a target TSH.
A 70 kg adult is being evaluated for thyroid hormone replacement.
Inputs:
Expected Result: Estimated dose = 1.6 × 70 = 112 µg/day — reported as the full replacement dose estimate.
Both use the same 1.6 µg/kg/day formula. Thyroid Dose provides the estimate, while Levothyroxine Dose includes clinical guidance on titration, cardiac precautions, and pregnancy adjustments.
In healthy patients, dose can be titrated every 6–8 weeks. In elderly or cardiac patients, increase more slowly (every 6–12 weeks).
Yes. Take 30–60 minutes before breakfast with water only. Separate from calcium, iron supplements, and PPIs.
Disclaimer: This calculator is intended for educational and clinical decision support purposes only. The result is not a prescription: actual starting doses are individualized, current treatment guidance and patient factors must be considered, and this calculator does not apply any automatic adjustment for age, cardiac disease, or pregnancy.
Clinical dosing with titration guidance.
Requires clinical context.