Levothyroxine Dose Calculator.

For an adult with overt hypothyroidism, the standard full-replacement dose of levothyroxine is about 1.6 micrograms per kilogram of body weight per day. A 70 kg person lands near 112 mcg — which is why 112 mcg is a common starting tablet. Subclinical hypothyroidism (a raised TSH with a still-normal free T4) usually needs less, often around 1.0 mcg/kg or a fixed low dose, because the thyroid is still doing part of the work.

Levothyroxine is dosed on actual body weight, not ideal weight, and it comes in an unusually fine range of tablet strengths (25, 50, 75, 88, 100, 112, 125, 137, 150, 175, 200 mcg) precisely because the therapeutic window is narrow and individual. This calculator rounds the weight-based figure to the nearest real tablet so you can see where a prescription is likely to start.

In adults over about 65, or anyone with coronary artery disease, guidelines advise against starting at the full weight-based dose. Thyroid hormone raises heart rate and myocardial oxygen demand, and loading a deconditioned or diseased heart too fast can provoke angina or arrhythmia. The safer path is to start at 25–50 mcg per day and step up every few weeks toward the target while symptoms and, if relevant, cardiac status are watched.

That is why, for the older/cardiac scenario, this tool shows a 25–50 mcg starting dose and presents the weight-based number only as the eventual goal to titrate toward — not the dose to begin with.

A weight-based estimate only gets you into the right neighborhood. The real dose is dialed in by measuring TSH about 6–8 weeks after starting or changing, and adjusting until TSH sits in the target range. Many things shift the requirement: pregnancy raises it substantially, malabsorption (celiac, bariatric surgery) raises it, and calcium, iron, and some foods block absorption if taken too close to the pill — which is why levothyroxine is taken on an empty stomach, well before other tablets.

Use this calculator to understand the ballpark and sanity-check a prescription, never to self-start or self-adjust. Too much levothyroxine over time carries real risks — atrial fibrillation and bone loss among them.