Levothyroxine Dosage Calculator
Enter your weight, height, age, and clinical scenario to get a weight-based levothyroxine starting dose following American Thyroid Association (ATA) guidelines. The calculator applies the standard 1.6 mcg/kg/day formula, then adjusts for obesity (using ideal body weight via the Devine formula), age over 65, cardiac disease, and five clinical scenarios. Results are rounded to the nearest available commercial tablet strength, with a two-tablet combination suggestion when a split helps hit the calculated target more closely. This tool is for educational reference only - your prescribing clinician determines your actual dose.
How the weight-based formula works
The standard starting dose for full thyroid hormone replacement in a healthy adult is 1.6 micrograms of levothyroxine per kilogram of body weight per day. This figure comes from the American Thyroid Association and the American Association of Clinical Endocrinologists, and it approximates how much thyroxine a healthy thyroid gland produces. For a 70 kg adult that comes to about 112 mcg/day, which maps neatly to a commercial tablet. The formula is a starting point only: TSH is rechecked 6-8 weeks later and the dose is adjusted in increments of 12.5 to 25 mcg until the target range is reached.
Why ideal body weight matters in obesity
Levothyroxine distributes into lean tissue rather than fat, so dosing on total body weight in a patient with a high BMI risks overshooting. When BMI is 30 or above, guidelines recommend using ideal body weight (IBW) instead. This calculator uses the Devine formula: IBW (female) = 45.5 + 2.3 x (height in inches over 60 inches); IBW (male) = 50 + 2.3 x (height in inches over 60 inches). The effective weight used in the calculation is the lower of actual and ideal body weight, which prevents the computed dose from climbing far above what lean tissue can safely absorb.
Special populations and dose caps
Two groups always start at a lower dose regardless of what the weight formula returns. First, patients aged 65 or older: the ATA recommends beginning at 25-50 mcg/day and titrating slowly, because excess thyroid hormone increases the risk of atrial fibrillation, angina, and bone loss in older adults. Second, patients with known cardiac disease, including ischemic heart disease, arrhythmia, or heart failure: the starting dose is capped at 12.5-25 mcg/day for the same cardiac reasons. In both cases the dose is increased every 6-8 weeks under close monitoring until TSH reaches the target range. Pregnancy is the opposite situation: the dose requirement rises by 20-30% in the first trimester, and TSH should be checked every four weeks until week 20.
Reading your TSH result and adjusting the dose
TSH is the most sensitive marker of thyroid hormone adequacy. For most adults the target range is 0.5 to 2.5 mIU/L, though some guidelines widen it to 4.0 mIU/L in older patients who feel well. If TSH remains high after 6-8 weeks, the dose is increased by 12.5 to 25 mcg; if it falls below range, the dose is decreased. Cancer patients on suppressive therapy aim for TSH below 0.1-0.5 mIU/L depending on recurrence risk. Subclinical hypothyroidism, defined as a high TSH with normal T3 and T4 levels, is sometimes watched rather than treated, especially in older patients where TSH can naturally drift higher. Levothyroxine has a half-life of about 7 days, so levels stabilise roughly 5-6 weeks after any dose change - which is why follow-up labs are scheduled at 6-8 week intervals.
Levothyroxine dose-per-kg by clinical scenario
| Clinical scenario | Dose rate (mcg/kg/day) | Target TSH (mIU/L) | Notes |
|---|---|---|---|
| New hypothyroidism diagnosis | 1.6 | 0.5 to 2.5 | Full replacement dose for healthy adults |
| Subclinical hypothyroidism | 1.0 to 1.2 | 1.0 to 2.5 | TSH 4-10 mIU/L; many guidelines defer treatment |
| Post-thyroidectomy (benign) | 1.7 | 0.5 to 2.0 | Slightly above replacement; monitor closely |
| Post-thyroidectomy (cancer) | 1.8 to 2.2 | 0.1 to 0.5 | TSH suppression reduces recurrence risk |
| Pregnancy | 2.0 to 2.4 | Below 2.5 (T1) | Dose demand rises 20-30% in the first trimester |
| Age >= 65 or cardiac disease | 25 to 50 mcg start | 0.5 to 3.0 | Start low, increase in 25 mcg steps every 6-8 weeks |
Typical weight-based starting dose rates from ATA and AACE guidelines. Final dose depends on age, cardiac history, and body composition.
Frequently asked questions
What is the standard levothyroxine starting dose?
For a healthy adult without cardiac disease or age-related concerns, the standard full-replacement starting dose is 1.6 mcg/kg/day. For a 70 kg person that is about 112 mcg/day. Elderly patients and those with heart conditions start at 25-50 mcg/day and titrate up slowly over several months.
Why does the calculator use ideal body weight instead of actual weight?
Levothyroxine distributes into lean body mass rather than adipose tissue. If actual body weight is used in a patient with obesity, the formula overestimates how much thyroid hormone the body will absorb, raising the risk of over-replacement (elevated heart rate, palpitations, bone loss). Ideal body weight, calculated with the Devine formula, more accurately reflects the lean compartment that the drug distributes into.
How often do I need to check my TSH after starting levothyroxine?
Most guidelines recommend rechecking TSH 6-8 weeks after starting or changing the dose, because levothyroxine has a 7-day half-life and levels take 5-6 weeks to stabilise. Once the dose is stable and you feel well, annual TSH testing is usually sufficient. Pregnancy is an exception - TSH should be checked every 4 weeks through the first trimester.
Why is the dose different for thyroid cancer versus routine hypothyroidism?
After thyroidectomy for thyroid cancer, the goal is not just to replace the missing hormone but also to suppress TSH below the normal range. TSH acts as a growth signal for any residual thyroid tissue or cancer cells, so keeping it low reduces the risk of recurrence. The dose rate for cancer patients is 1.8-2.2 mcg/kg/day, compared to 1.6 mcg/kg/day for routine replacement, and the target TSH is 0.1-0.5 mIU/L for intermediate-risk patients and below 0.1 mIU/L for high-risk patients.
Does it matter when I take levothyroxine?
Yes. Levothyroxine is best absorbed on an empty stomach. The most common recommendation is to take it first thing in the morning, 30-60 minutes before food and other medications. Calcium supplements, iron tablets, antacids, and some cholesterol drugs all reduce absorption if taken at the same time - space them at least 4 hours apart. Evening dosing is acceptable if taken at least 3-4 hours after your last meal.
What are the available levothyroxine tablet strengths?
Standard commercially available strengths are 25, 50, 75, 88, 100, 112, 125, 137, 150, 175, and 200 mcg. If your ideal dose falls between two strengths, you can alternate tablets on different days - for example, alternating 100 mcg and 112 mcg gives a mean of 106 mcg per day. Some compounding pharmacies and liquid formulations (such as Tirosint-SOL) offer finer increments if needed.
Is this calculator a substitute for a doctor's prescription?
No. This tool provides an educational estimate based on published dosing guidelines. Your actual prescription depends on your complete medical history, recent TSH and free T4 results, other medications, and your clinician's clinical judgment. Always have thyroid hormone levels measured before starting therapy and follow up with your prescribing provider.