How this instrument works
Levothyroxine is the synthetic thyroid hormone used to treat hypothyroidism, and the dose that eventually keeps a patient's thyroid-stimulating hormone (TSH) in range is fairly predictable from body weight. The American Thyroid Association's 2014 guideline on treating hypothyroidism gives a full weight-based replacement dose of about 1.6 mcg per kilogram per day for an otherwise healthy adult — this calculator applies that figure directly to a weight in kilograms to estimate the eventual full-replacement target.
Starting at that full target dose on day one isn't always the right move, though. In younger, otherwise healthy patients it usually is — the guideline supports starting directly at the calculated full weight-based dose. But in patients over 50, or anyone with known cardiac disease, suddenly correcting hypothyroidism at full strength can unmask or worsen angina and other cardiac symptoms, because thyroid hormone increases the heart's oxygen demand. For that group, the guideline instead recommends starting low — a flat, conservative dose around 25 mcg/day regardless of weight — and titrating upward gradually.
This calculator does the weight-based arithmetic and applies the age/cardiac-risk branch point from the guideline; it does not replace clinical judgment. Actual dosing requires a baseline TSH (and often free T4), a review of other conditions and medications, and periodic re-checks of TSH roughly every 6 to 8 weeks after any dose change or new start, with the dose adjusted based on those results until TSH stabilizes in range.
- Enter the patient's weight in kilograms.
- Select whether the patient is over 50 years old, or has known cardiac disease (Yes/No).
- Read the full weight-based replacement dose in mcg/day (≈1.6 mcg/kg/day).
- Read the recommended starting dose in mcg/day — this equals the full dose unless the age/cardiac flag is Yes, in which case it drops to a flat 25 mcg/day starting point.
Worked example — 70 kg adult, with and without cardiac risk
A 70 kg adult with no cardiac history and under 50 has a full weight-based replacement dose of 70 × 1.6 = 112 mcg/day, and the guideline supports starting directly at that full dose — so both the full-replacement figure and the recommended starting dose come out to 112 mcg/day.
The same 70 kg patient, but over 50 or with known cardiac disease, still has a full-replacement target of 112 mcg/day — the weight-based number doesn't change — but the recommended starting dose drops to a flat 25 mcg/day instead. The clinician then titrates upward from that low starting point over subsequent visits, guided by TSH results, rather than jumping straight to 112 mcg/day.
A smaller, lower-risk 50 kg adult's full weight-based dose comes out lower too: 50 × 1.6 = 80 mcg/day, which is also the recommended starting dose in the absence of age or cardiac risk factors.
Questions
Is this the dose I should actually take or prescribe?
Treat it only as a reference starting point. The American Thyroid Association guideline this calculator is based on gives population-level weight-based estimates, but the right dose for a specific patient depends on the cause and severity of hypothyroidism, baseline TSH and free T4, other medications and conditions, and how the patient responds on repeat labs — all of which require a clinician's evaluation, not a weight-only calculation.
Why does age or cardiac disease change the starting dose so much?
Levothyroxine increases the body's metabolic rate and the heart's oxygen demand. Starting an older patient or one with known cardiac disease at a full weight-based dose can unmask or worsen angina, arrhythmias, or other cardiac symptoms, so the ATA guideline instead recommends a conservative flat starting dose — commonly around 25 mcg/day — with slow upward titration rather than jumping straight to the full replacement target.
How often should the dose be adjusted after starting?
The ATA guideline recommends rechecking TSH about every 6 to 8 weeks after starting or changing a dose, since it takes roughly that long for levothyroxine levels and TSH to fully equilibrate at a new dose. The dose is then adjusted in small increments based on that result until TSH settles into the target range, at which point monitoring intervals typically lengthen.
Why 1.6 mcg per kilogram specifically?
That figure is the full weight-based replacement dose estimate given in the American Thyroid Association's 2014 hypothyroidism treatment guideline for adults with complete loss of thyroid function, based on the dose typically needed to normalize TSH across studied populations. Individual patients can still need somewhat more or less depending on residual thyroid function, absorption, and other factors — it's a population estimate, not a guarantee for any one person.
Does this calculator account for pregnancy, other medications, or malabsorption?
No. Those all meaningfully change levothyroxine dosing — pregnancy typically increases requirements, and drugs like calcium, iron, proton pump inhibitors, and some other medications can interfere with absorption or metabolism. This tool only performs the weight-based and age/cardiac-risk arithmetic described in the guideline; a full dosing decision needs a clinician who knows the patient's complete history.
What if the patient's weight changes significantly over time?
Because the full-replacement target scales with weight, a significant weight change is a reasonable trigger to recheck TSH and reassess the dose with a clinician, rather than assuming the original dose still fits. The calculator only reflects the weight entered at the time you use it.
References
- Jonklaas J, Bianco AC, Bauer AJ, et al. — Guidelines for the Treatment of Hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement. Thyroid. 2014;24(12):1670-1751 (PMC free full text)
- American Thyroid Association — ATA Professional Guidelines
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.