Levothyroxine Dosage Calculator
Estimate starting levothyroxine dose from age, weight, height, FT4 level, and heart disease status.
Starting Levothyroxine for Hypothyroidism
Levothyroxine replaces missing thyroxine (T4) in hypothyroidism. Most adults need a daily dose based on lean or ideal body weight rather than total weight, because adipose tissue does not metabolize thyroid hormone like lean tissue. This calculator estimates a starting dose from sex, age, weight, height, FT4 level, and heart disease status. For related metabolic tools, see the BMR Calculator.
Weight-Based Dosing
Clinical hypothyroidism with low FT4 is often started near 1.6 mcg/kg per day in younger adults. Subclinical disease or normal FT4 usually warrants a lower starting point near 1.0 mcg/kg. When actual weight exceeds Devine ideal body weight, the calculator uses ideal weight to avoid overdosing overweight patients.
Special Populations
Elderly patients and those with heart disease need cautious initiation. Cardiac patients typically start at 25 mcg/day regardless of size. Adults aged 60 and older are usually capped near 50 mcg/day before titration. Recheck TSH 6 to 8 weeks after starting or changing dose.
Frequently Asked Questions
How is the levothyroxine starting dose calculated?
Multiply dosing weight (actual or Devine ideal weight if overweight) by 1.6 mcg/kg for clinical hypothyroidism with low FT4, or 1.0 mcg/kg for subclinical or normal FT4. The result is rounded to a common tablet strength.
Why use ideal body weight instead of actual weight?
Excess fat tissue contributes little to thyroid hormone needs. Using actual weight in obese patients can overestimate dose and increase risks such as palpitations or atrial fibrillation.
What starting dose is used with heart disease?
Patients with cardiovascular disease should begin at 25 mcg/day and increase slowly in 12.5 to 25 mcg steps every 4 to 8 weeks based on TSH and symptoms.
What TSH target should I aim for?
Younger adults often target about 0.5 to 2.0 mU/L. Older adults may aim for about 2.0 to 4.0 mU/L. Always follow your lab's reference range and clinician guidance.
Does this replace medical advice?
No. Levothyroxine dosing must be individualized and monitored by a healthcare provider. Use this tool for education only, not to start or change prescribed therapy.
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