Levothyroxine is the standard treatment for hypothyroidism, and getting the starting dose right matters: too low and the patient stays symptomatic for longer than necessary, too high (especially in older or cardiac patients) can precipitate angina or arrhythmia. This calculator estimates the full-replacement target from body weight and applies age/cardiac-risk guidance to recommend a safer starting point.
How the full-replacement dose is calculated
The standard full-replacement dose is about 1.6 mcg of levothyroxine per kilogram of body weight per day, per the American Thyroid Association's 2014 Guidelines for the Treatment of Hypothyroidism. This is the dose expected to normalize TSH at steady state in most adults — a target, not automatically a safe starting dose.
Why older and cardiac patients start lower
Starting a patient at the full calculated replacement dose can unmask or worsen coronary ischemia and precipitate arrhythmia in older adults or those with cardiac disease, because thyroid hormone increases cardiac oxygen demand. Standard teaching is to "start low and go slow": begin at a fixed, lower dose (commonly 25-50 mcg/day) and increase gradually — by 12.5-25 mcg every 6-8 weeks — based on repeat TSH, rather than starting at the weight-based target directly.
Limits and edge cases
This calculator is scoped to adults (18 and older); it deliberately does not compute a pediatric dose, since pediatric and neonatal hypothyroidism dosing follows separate age-band mcg/kg/day tables with much higher per-kilogram doses in infants. It also does not account for pregnancy (which typically increases requirements), malabsorption, or drug interactions (calcium, iron, and proton-pump inhibitors can all reduce absorption) — the calculator flags unusually high calculated doses so these can be investigated, but the underlying cause requires clinical evaluation. Titration is always driven by repeat TSH, not by a fixed timeline alone.