Levothyroxine Dose Calculator

Q: What is the standard levothyroxine replacement dose?

The standard full-replacement dose for adult hypothyroidism is about 1.6 mcg/kg/day of body weight, per the American Thyroid Association's 2014 Guidelines for the Treatment of Hypothyroidism. This is a target maintenance dose, not necessarily the starting dose.

Q: Why do older adults start at a lower dose?

In patients aged 60-69 or with known cardiac disease, starting at the full calculated replacement dose can precipitate angina or arrhythmia. Guidelines recommend a lower fixed starting dose (commonly 25-50 mcg/day) with gradual titration, rather than jumping straight to the weight-based target.

Q: How is the dose titrated?

After starting, TSH is rechecked in about 6-8 weeks and the dose is adjusted by 12.5-25 mcg/day increments until TSH reaches the target range. This calculator shows the starting-dose guidance only — titration itself is a clinical, TSH-driven process.

Q: Why doesn't this calculator compute a pediatric dose?

Pediatric and neonatal hypothyroidism dosing follows separate age-band mcg/kg/day tables with higher per-kilogram doses in infants that taper with age — a distinct clinical domain from adult replacement dosing. This calculator is scoped to adults (18 and older); pediatric dosing must be determined by a pediatrician or pediatric endocrinologist.

Q: Why does the calculator round to a tablet strength?

Levothyroxine is dispensed in fixed commercial tablet strengths (for example 25, 50, 88, 100, 112, 125, 150 mcg). A calculated target dose that falls between two strengths is commonly managed clinically by alternating two tablet strengths across the week to approximate the target average — this calculator shows both the nearest single strength and that alternate-day pattern.

Q: Is this calculator a substitute for a prescriber?

No. This tool is for clinical/educational reference only. Levothyroxine dosing must be determined and monitored by a licensed prescriber using TSH-guided titration, and every dose confirmed against current prescribing information before administering.

Estimate a full-replacement or age-adjusted starting levothyroxine dose from body weight, per American Thyroid Association guidelines.

Patient Inputs

Body weight

Age (years)

Adults 18 and older only. Pediatric dosing uses separate age-specific guidance.

Known cardiac disease (e.g. coronary artery disease, arrhythmia)

Age 60 or older, or a positive cardiac history, lowers the recommended starting dose below the full calculated replacement dose.

Dose weight basis

Ideal-weight mode requires height and sex.

Treatment indication

Severe or longstanding untreated hypothyroidism

Pregnant or trying to conceive

Known or suspected adrenal insufficiency

Estimated Starting-Dose Range

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Educational estimate — confirm with prescriber

Risk tier: —

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Full replacement target (1.6 mcg/kg/day)

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Risk-adjusted starting dose

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Nearest tablet strength

Starting Dose by Risk Tier

Older adults and patients with cardiac disease should not start levothyroxine at the full calculated replacement dose — doing so risks angina or arrhythmia. The bars below compare the starting dose for each risk tier; the tier that matches the Dose Estimate tab's inputs is highlighted. Enter weight and age on that tab to populate this comparison.

Low risk (age < 60, no cardiac history) — full replacement dose

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Moderate risk (age 60–69, no cardiac history)

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High risk (cardiac history, or age 70+)

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Titration: Enter weight and age on the Dose Estimate tab to see titration guidance.

Which tier applies? Low risk (younger, no cardiac history) can typically start at the full calculated replacement dose directly. Moderate and high risk patients start at a fixed lower dose and titrate upward under TSH monitoring — the goal is the same target replacement dose, reached more cautiously.

⚕ For educational purposes only. Not a substitute for a prescriber's judgment.

Tablet Strength Rounding

Levothyroxine is dispensed in fixed commercial tablet strengths. When a calculated target dose falls between two strengths, clinicians commonly alternate two tablet strengths across the week so the weekly average approximates the target.

Target	Calculated dose	Nearest tablet / alternate-day plan
Enter weight and age on the Dose Estimate tab to see tablet rounding.

Standard Tablet Strengths

Available strength

Source: Standard commercial levothyroxine tablet strengths (25, 50, 75, 88, 100, 112, 125, 137, 150, 175, 200, 300 mcg), as manufactured for common branded and generic products.

⚕ Educational reference only. Tablet selection and any alternate-day dosing pattern must be confirmed with a pharmacist or prescriber.

3 min read 3 steps 5 terms 3 examples 6 FAQs fullReplacement = weight(kg) x 1.6 mcg/kg/day; startingDose …

High YMYL Reviewed by Calculover Clinical Team · Updated 2026-07-23 · 2 sources

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Walk-through

How to Use This Calculator

3 steps

Enter body weight, age, and cardiac history

On the Dose Estimate tab, enter body weight (in kg or lb), age, and whether the patient has known cardiac disease. The calculator computes the full replacement target (1.6 mcg/kg/day) and a risk-adjusted starting dose instantly as you type.

Read the recommended starting dose and risk tier

The result card shows the recommended starting dose, the risk tier (low, moderate, or high), the full-replacement target, and the nearest available tablet strength. Patients under 60 without cardiac disease start at the full replacement dose; older or cardiac patients start lower and titrate up.

Compare risk tiers and round to a tablet strength

Use the Age-Adjusted Starting Dose tab to see how the starting dose differs across risk tiers and the titration schedule, and the Tablet Rounding tab to see the nearest commercial tablet strength (or alternate-day pattern) for the calculated dose. Always confirm the plan with a prescriber before use.

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Reference

Formula & Methodology

1 formula

Full-replacement dose and risk-tiered starting dose

fullReplacement = weight(kg) x 1.6 mcg/kg/day; startingDose = fullReplacement if low-risk, else a fixed 25-50 mcg/day

The full-replacement target is 1.6 mcg/kg/day of body weight, per the American Thyroid Association's 2014 Guidelines for the Treatment of Hypothyroidism. Patients under 60 with no cardiac history can typically start at this full dose. Patients aged 60-69 start at a fixed 50 mcg/day, and patients with cardiac disease or aged 70+ start at a fixed 25 mcg/day — both groups then titrate upward by 12.5-25 mcg every 6-8 weeks based on repeat TSH until the full target is reached.

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High YMYL

Trust, Methodology & Sources

Reviewed by Calculover Clinical Team · Updated 2026-07-23 · 2 sources

Editorial accountability

Author: Calculover Clinical Team - Clinical content team

Owner: Calculover Editorial - Editorial oversight

Reviewer: Calculover Clinical Team - Clinical reviewer

Last reviewed: 2026-07-23

Last verified: 2026-07-23

Data effective: 2026-07-23

Methodology

Estimates the full-replacement levothyroxine dose as 1.6 mcg/kg/day of body weight, per the American Thyroid Association's 2014 Guidelines for the Treatment of Hypothyroidism (Jonklaas J, et al. Thyroid. 2014). A three-tier risk model then sets the recommended STARTING dose: low risk (age under 60, no cardiac history) starts at the full calculated dose; moderate risk (age 60-69, no cardiac history) starts at a fixed 50 mcg/day; high risk (cardiac history, or age 70+) starts at a fixed 25 mcg/day. Both moderate- and high-risk tiers then titrate upward by 12.5-25 mcg every 6-8 weeks based on repeat TSH. The calculator is scoped to adults only and hard-blocks any input under age 18, since pediatric dosing follows separate age-band tables.

Assumption: Body weight is entered as current actual body weight; the calculator does not distinguish ideal vs. actual body weight in obese patients, which some clinicians adjust for.

Assumption: The three-tier starting-dose model is a simplified, commonly taught approximation of 'start low, go slow' guidance and does not replace an individualized clinical risk assessment.

Assumption: Titration is TSH-driven in practice; the 6-8 week interval and 12.5-25 mcg increment shown are typical guidance, not a rigid protocol.

Limitations & guidance

This calculator does not compute a pediatric or neonatal dose; ages under 18 are hard-blocked and must be referred to pediatric-specific dosing tables.

It does not account for pregnancy, malabsorption, or drug interactions (e.g. calcium, iron, proton-pump inhibitors) that can change the effective dose — it only flags unusually high calculated full-replacement doses for further evaluation.

The calculator is for clinical decision support and education only and does not replace a licensed prescriber's judgment, TSH-guided titration, or current prescribing information.

Professional guidance: This calculator is for clinical decision support and educational purposes only and is not a substitute for professional medical judgment, diagnosis, or treatment. Verify all doses with a pharmacist or current prescribing information before administering, and use TSH-guided titration under a licensed prescriber's supervision.

Primary sources

Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the Treatment of Hypothyroidism. American Thyroid Association. Thyroid. 2014;24(12):1670-1751.

American Thyroid Association. Hypothyroidism (Patient Brochure).

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Glossary

Key Terms Explained

5 terms

Full-replacement dose The steady-state maintenance dose expected to normalize TSH once titration is complete — about 1.6 mcg/kg/day of body weight in most adults. It is a target, not necessarily a safe starting dose for every patient.

Starting dose The dose used to begin therapy. In healthy, younger adults it can equal the full-replacement dose. In older adults or those with cardiac disease, the starting dose is deliberately lower to avoid precipitating angina or arrhythmia, then increased gradually.

TSH (thyroid-stimulating hormone) The blood test used to guide levothyroxine titration. Dose adjustments are made in response to a repeat TSH drawn roughly 6-8 weeks after a dose change, not on a fixed schedule alone.

Euthyroid The state of having thyroid hormone levels within the normal range. The goal of titration is to reach and maintain a euthyroid TSH, which may take several dose adjustments over months.

Alternate-day dosing A practical strategy for hitting a target dose that falls between two commercial tablet strengths: alternating two strengths across the week (e.g. six days at one strength, one day at the next) so the weekly average approximates the target.

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Scenarios

Real-World Examples

3 worked examples

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Healthy younger adult

70 kg, age 45, no cardiac history

Body weight 70 kg Age 45 Cardiac history No

70 kg x 1.6 mcg/kg/day = 112 mcg/day full-replacement target. Low risk (age under 60, no cardiac history), so the recommended starting dose is the full 112 mcg/day, which is also a standard commercial tablet strength — no rounding needed.

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Older adult with cardiac history

65 kg, age 74, known coronary artery disease

Body weight 65 kg Age 74 Cardiac history Yes

65 kg x 1.6 mcg/kg/day = 104 mcg/day full-replacement target, but cardiac history and age 70+ place this patient in the high-risk tier. The recommended starting dose is a fixed 25 mcg/day, titrated upward by 12.5-25 mcg every 6-8 weeks based on repeat TSH — not a direct jump to the 104 mcg/day target.

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Pediatric patient (out of scope)

Age 10

Body weight 32 kg Age 10 Cardiac history No

No dose is shown. This calculator is scoped to adults (18 and older); pediatric levothyroxine dosing follows separate age-band mcg/kg/day tables that this tool does not model. A pediatrician or pediatric endocrinologist should determine the pediatric dose.

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Reference

Cite This Calculator

APA & MLA

Use either format to cite this calculator in a paper, report, or resource list.

APA

MLA

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Deep Dive

Understanding Levothyroxine Dosing

3 chapters · 3 min read

Levothyroxine is the standard treatment for hypothyroidism, and getting the starting dose right matters: too low and the patient stays symptomatic for longer than necessary, too high (especially in older or cardiac patients) can precipitate angina or arrhythmia. This calculator estimates the full-replacement target from body weight and applies age/cardiac-risk guidance to recommend a safer starting point.

How the full-replacement dose is calculated

The standard full-replacement dose is about 1.6 mcg of levothyroxine per kilogram of body weight per day, per the American Thyroid Association's 2014 Guidelines for the Treatment of Hypothyroidism. This is the dose expected to normalize TSH at steady state in most adults — a target, not automatically a safe starting dose.

Why older and cardiac patients start lower

Starting a patient at the full calculated replacement dose can unmask or worsen coronary ischemia and precipitate arrhythmia in older adults or those with cardiac disease, because thyroid hormone increases cardiac oxygen demand. Standard teaching is to "start low and go slow": begin at a fixed, lower dose (commonly 25-50 mcg/day) and increase gradually — by 12.5-25 mcg every 6-8 weeks — based on repeat TSH, rather than starting at the weight-based target directly.

Limits and edge cases

This calculator is scoped to adults (18 and older); it deliberately does not compute a pediatric dose, since pediatric and neonatal hypothyroidism dosing follows separate age-band mcg/kg/day tables with much higher per-kilogram doses in infants. It also does not account for pregnancy (which typically increases requirements), malabsorption, or drug interactions (calcium, iron, and proton-pump inhibitors can all reduce absorption) — the calculator flags unusually high calculated doses so these can be investigated, but the underlying cause requires clinical evaluation. Titration is always driven by repeat TSH, not by a fixed timeline alone.

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Questions

Frequently Asked Questions

6 questions

What is the standard levothyroxine replacement dose?+