Levothyroxine Dose Calculator.
Estimate a weight-based levothyroxine (thyroid hormone) starting dose using the standard ~1.6 mcg/kg rule. This is a sanity-check against the textbook figure. Your actual dose is set and adjusted by your clinician using your TSH.
Frequently Asked Questions
How is a levothyroxine dose calculated by weight?+
Full replacement is conventionally about 1.6 micrograms per kilogram of body weight per day. For an 80 kg adult that is roughly 128 mcg, which then gets rounded to a strength that actually exists as a tablet. This calculator applies that factor and does the rounding.
What is the levothyroxine dose per kg?+
Around 1.6 mcg/kg/day for full replacement: overt hypothyroidism, or after thyroid removal. Subclinical hypothyroidism, where TSH is raised but free T4 is still normal, typically uses far less, nearer 1.0 mcg/kg/day, and is often started as a fixed low dose and titrated instead.
Why does the calculator round my dose?+
Because levothyroxine is only manufactured in specific strengths: 25, 50, 75, 88, 100, 112, 125, 137, 150, 175 and 200 mcg. A weight-based figure of 128 mcg is not dispensable, so the tool shows the nearest real tablet rather than a number no pharmacy can fill.
Should an older adult or someone with heart disease start at the full weight-based dose?+
No. Over roughly 65, or with known coronary disease, guidelines advise starting at 25 to 50 mcg per day and increasing slowly, because raising thyroid hormone too quickly increases cardiac demand. In that situation the weight-based number is the eventual target, not the starting point. The calculator flags this explicitly when you choose that scenario.
How is the dose adjusted after starting?+
By blood test, not by weight. TSH is rechecked about six to eight weeks after any change, because that is how long the level takes to settle, and the dose is moved in small steps until TSH sits where your clinician wants it. Weight-based dosing only ever sets the opening estimate.
What changes how much levothyroxine I need?+
Pregnancy raises the requirement substantially. Malabsorption conditions such as coeliac disease, and other medications (notably calcium, iron and some antacids) reduce how much you absorb, which is why levothyroxine is taken on an empty stomach and away from those. Absorption, not just body weight, determines the dose that works.
Is this a prescription?+
No. It is a weight-based estimate for orientation only. Levothyroxine is titrated to your own blood results by a clinician, and starting or changing a dose without one is genuinely unsafe.
The weight-based starting rule.
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.
Why older hearts start low and go slow.
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.
The dose is set by your TSH, not a formula.
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.
- Jonklaas J, Bianco AC, Bauer AJ, et al. (2014). Guidelines for the treatment of hypothyroidism (American Thyroid Association Task Force). Thyroid, 24(12), 1670–1751.
- Garber JR, Cobin RH, Gharib H, et al. (2012). Clinical practice guidelines for hypothyroidism in adults (AACE/ATA). Endocr Pract, 18(6), 988–1028.
For education, not medical advice. Results are estimates, not a diagnosis. Discuss any abnormal value or health concern with a qualified clinician.
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