Thyroid Hormone Dose Calculator

Levothyroxine replacement is anchored to a weight-based estimate because thyroxine distributes with lean body mass and is cleared at a fairly predictable fractional rate. Full replacement for primary hypothyroidism is about 1.6 micrograms per kilogram per day, so a 65 kg adult needs roughly 100 micrograms. The figure falls to about 1.0 to 1.1 micrograms per kilogram in people over 70, and lower still when ischemic heart disease makes cautious initiation necessary. Suppressive therapy after differentiated thyroid cancer uses 2.0 to 2.5 micrograms per kilogram to push TSH below 0.1 mIU/L in high-risk disease and to 0.1 to 0.5 mIU/L at intermediate risk, while ordinary replacement targets a TSH of roughly 0.5 to 4.0 mIU/L. Because levothyroxine has a half-life near seven days, a dose change needs 6 to 8 weeks to reach steady state, and TSH checked earlier is misleading. Tablets come in 25, 50, 75, 88, 100, 112, 125, 137, 150, 175 and 200 microgram strengths, so calculated doses are rounded to the nearest available strength or achieved by alternating two strengths across the week. The estimate has real limits. Total body weight overestimates the dose in obesity because adipose tissue contributes little to thyroxine clearance, absorption falls 20 to 40 percent when the tablet is taken with food and is reduced further by calcium, iron, proton pump inhibitors and bile acid sequestrants, and pregnancy raises requirements by 25 to 50 percent. A weight-based number is a starting point; TSH-guided titration determines the final dose.

Treatment of hypothyroidism began in 1891, when George Redmayne Murray of Newcastle injected an extract of sheep thyroid into a woman with myxedema and reported her recovery in the British Medical Journal. Oral thyroid extract soon replaced injection, and desiccated animal thyroid remained standard for the next seventy years. Edward Kendall crystallized thyroxine at the Mayo Clinic in 1914, and Charles Harington and George Barger at University College London determined its structure and achieved its synthesis in 1926 and 1927, making a pure single-hormone preparation chemically possible. Synthetic sodium levothyroxine became widely available during the 1960s, but dosing stayed empirical until radioimmunoassay for TSH, developed in the 1960s and refined into sensitive second and third generation assays through the 1980s, allowed replacement to be titrated against pituitary feedback rather than against symptoms. The sensitive assays also revealed that prevailing doses were frequently excessive, and average replacement fell substantially toward the 1.6 micrograms per kilogram figure used today. Later guidance from the American Thyroid Association, including the 2014 hypothyroidism treatment guidelines and the 2015 thyroid nodule and differentiated thyroid cancer guidelines, formalized weight-based starting doses, cautious initiation in elderly and cardiac patients, and risk-stratified TSH suppression targets.