Thyroid Dose Calculator

Levothyroxine replacement dosing starts as a weight-based calculation and is then refined against the TSH response. A young, healthy adult with primary hypothyroidism can usually start close to the full estimated replacement dose, calculated at roughly 1.6 micrograms per kilogram of body weight per day, the starting dose recommended by the American Thyroid Association's 2014 guidelines for the treatment of hypothyroidism. Elderly patients and anyone with known or suspected cardiac disease are treated differently: because levothyroxine can unmask or worsen angina and arrhythmias by raising metabolic demand, this calculator starts them at a flat 25 micrograms per day regardless of their weight-based target, to be titrated upward gradually rather than dosed to target immediately.

Either way, the calculated dose is rounded to the nearest commercially available tablet strength, since levothyroxine tablets come in fixed increments rather than continuous doses. TSH and free T4 are read independently for interpretation and dose-adjustment guidance at follow-up: TSH between 0.4 and 4.0 mIU/L is the euthyroid target range, 4.0-10.0 mIU/L suggests a modest dose increase is needed, and above 10.0 mIU/L suggests a larger increase, while TSH below 0.4 mIU/L suggests the dose should come down. Because TSH lags a levothyroxine dose change by roughly six to eight weeks to fully equilibrate, this calculator's dose-adjustment guidance is meant for that recheck interval, not for changing doses more frequently than that.