Levothyroxine Dosage Calculator
Estimate your thyroid replacement therapy starting dose based on weight and clinical status.
Estimated Daily Dosage
Based on your input, the calculated daily dose is:
How is Levothyroxine Dosage Calculated?
Levothyroxine (synthetic T4) is primarily dosed based on a patient's lean body mass and the degree of thyroid deficiency. For patients with overt hypothyroidism (where the thyroid produces very little to no hormone), the standard full replacement dose is approximately 1.6 micrograms (mcg) per kilogram (kg) of body weight per day.
However, this is not a "one size fits all" calculation. Several factors can influence the starting dose:
- Age: Patients over 60 often require 20-25% less medication because of reduced metabolic rates.
- Cardiac Health: For those with coronary artery disease or arrhythmias, doctors typically start with a very low dose (12.5 to 25 mcg) to avoid straining the heart.
- Pregnancy: Thyroid hormone requirements increase significantly during pregnancy (often by 30-50%) to support fetal development.
- Severity: Those with subclinical hypothyroidism (elevated TSH but normal T4) may only require a partial replacement dose, often starting at 25-75 mcg daily.
Typical Standard Tablet Sizes
Because levothyroxine is manufactured in specific increments, your doctor will likely round your calculated dose to the nearest available tablet size. Common strengths include:
25, 50, 75, 88, 100, 112, 125, 137, 150, 175, and 200 mcg.
Practical Examples
Example 1: Healthy Adult
A 70 kg (154 lbs) adult with no cardiac issues requires full replacement.
Calculation: 70 kg × 1.6 mcg/kg = 112 mcg.
The patient would likely be prescribed 112 mcg daily.
Example 2: Elderly Patient with Heart History
A 80 kg (176 lbs) patient over 70 years old with a history of heart disease.
Calculation: 80 kg × 0.5 mcg/kg = 40 mcg.
To be safe, the doctor might start with a 25 mcg or 37.5 mcg dose and titrate up slowly.
Monitoring and Adjustments
After starting levothyroxine, it takes about 6 to 8 weeks for thyroid hormone levels to stabilize in the blood. Doctors will typically order a TSH blood test at this interval to see if the dose needs to be adjusted. The goal is to bring the TSH within the "euthyroid" range (typically 0.4 to 4.0 mIU/L, though targets vary by age and pregnancy status).