Levothyroxine Dosage Calculator

Estimate a weight-based levothyroxine full-replacement dose and a safer starting dose, then round both to real tablet strengths using the standard mcg-per-kilogram formula.

Quick Facts

Full replacement
~1.6 mcg per kg body weight per day
Average adult full-replacement dose per standard endocrinology guidelines.
Start low, go slow
25-50 mcg/day starting dose
Used for patients 50+ or with cardiac disease, titrated upward over time.
Recheck timing
TSH at 6-8 weeks
Levothyroxine's long half-life means dose changes take weeks to show full effect.

Your Results

Calculated
Calculated full dose
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Weight x mcg/kg factor
Recommended starting dose
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Rounded to nearest tablet
Titration target
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Full dose, nearest tablet
Starting dose, weekly
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7-day total

Ready

Enter body weight, indication, and cardiac risk, then calculate.

About the levothyroxine dosage formula

Levothyroxine (synthetic T4) is dosed by body weight because it replaces a hormone the body normally produces in proportion to metabolic mass. Standard endocrinology guidance expresses the target as a mcg-per-kilogram-per-day factor, multiplied by the patient's body weight, then rounded to a tablet strength that actually exists. This calculator performs that arithmetic; it does not replace a clinician's judgment or a measured TSH.

How the calculation works

The full-replacement dose is body weight in kilograms multiplied by a mcg/kg factor that depends on the treatment goal: about 1.6 mcg/kg/day for full replacement of primary hypothyroidism, about 1.0 mcg/kg/day for subclinical hypothyroidism (a smaller deficit), and about 2.2 mcg/kg/day for TSH-suppressive therapy after thyroidectomy for thyroid cancer. That raw number is then rounded to the nearest tablet strength that levothyroxine is actually manufactured in.

Why the starting dose can be lower than the target

  • Patients under 50 with no known heart disease are typically started at or near the full calculated dose.
  • Patients 50 or older, or anyone with known cardiac disease, are conventionally started lower - around 25 to 50 mcg per day - because a large jump in thyroid hormone can provoke angina or arrhythmia.
  • The dose is then titrated upward in small steps (commonly 12.5 to 25 mcg) toward the calculated target, guided by follow-up TSH results rather than the formula alone.

Important disclaimer

This tool performs a standard dosing calculation for education and planning. It is not a prescription and does not diagnose or treat any condition. Levothyroxine dosing must be confirmed and adjusted by a licensed clinician using actual TSH and free T4 results, symptoms, and other medications.

Frequently Asked Questions

What formula does this levothyroxine dosage calculator use?
It uses the standard weight-based replacement formula: dose (mcg/day) = body weight in kilograms x a mcg/kg factor. The factor is about 1.6 mcg/kg/day for full replacement of primary hypothyroidism, about 1.0 mcg/kg/day for subclinical hypothyroidism, and about 2.2 mcg/kg/day for TSH-suppressive dosing after thyroidectomy for thyroid cancer. The calculated dose is then rounded to the nearest available tablet strength.
Why does the calculator suggest a lower starting dose for some patients?
Patients age 50 and older or with known cardiac disease are conventionally started on a lower dose, around 25 to 50 mcg per day, rather than the full calculated replacement dose, because a large initial dose can provoke angina or arrhythmia in susceptible patients. The dose is then titrated upward in small steps toward the calculated target based on follow-up TSH results.
How is the levothyroxine dose adjusted after starting?
TSH is typically rechecked about 6 to 8 weeks after starting or changing a dose, since levothyroxine has a long half-life and takes that long to reach steady state. The dose is then adjusted in increments of about 12.5 to 25 mcg per day based on whether TSH is above or below the target range, and this calculator's numbers are a starting estimate for that process, not a final prescription.

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