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The levothyroxine dose calculator estimates the appropriate starting or target dose of levothyroxine (L-T4, synthetic thyroxine) for thyroid hormone replacement in hypothyroidism. Levothyroxine is the most commonly prescribed endocrine medication in most developed countries, used for primary hypothyroidism, secondary hypothyroidism, post-thyroidectomy replacement, and thyroid cancer suppressive therapy. The full replacement dose for primary hypothyroidism in young healthy adults is typically 1.6 mcg/kg/day of ideal body weight, though this is a starting estimate — actual requirements vary significantly between individuals depending on age, body composition, aetiology of hypothyroidism (partial vs complete thyroid failure), absorption, and drug interactions. In elderly patients and those with ischaemic heart disease, a cautious starting dose of 12.5-25 mcg/day is recommended, increasing by 12.5-25 mcg every 4-8 weeks to avoid precipitating cardiac ischaemia or arrhythmia. In patients with central hypothyroidism (pituitary or hypothalamic cause), TSH cannot be used as a dose titration target — FT4 (targeting the mid-normal range) and clinical symptoms guide dosing. After total thyroidectomy for differentiated thyroid cancer (DTC), TSH suppression targets depend on risk stratification: high-risk disease targets TSH 0.1-0.5 mU/L; low-risk DTC targets TSH in the low-normal range 0.5-2.0 mU/L. Levothyroxine has a narrow therapeutic window and small dose changes (even 25 mcg) can produce substantial TSH changes due to the logarithmic TSH-FT4 relationship. Absorption is affected by calcium, iron, PPIs, and fibre — levothyroxine should be taken on an empty stomach 30-60 minutes before food or other medications.
Full replacement dose = 1.6 mcg/kg/day (ideal body weight); Elderly/cardiac start: 12.5-25 mcg/day; Post-thyroidectomy for cancer: weight-based + TSH target (suppressive: 0.1-0.5 mU/L for high-risk; low-normal: 0.5-2.0 mU/L for low-risk DTC)
- 1Determine the indication: primary hypothyroidism (overt vs subclinical), central hypothyroidism, post-thyroidectomy replacement, or differentiated thyroid cancer suppressive therapy.
- 2Assess patient factors: age, body weight (use ideal body weight for obese patients), cardiac history, and absorption issues.
- 3For overt primary hypothyroidism in young healthy adults: calculate starting dose as 1.6 mcg/kg/day x ideal body weight, rounding to nearest available tablet strength (typical tablets: 25, 50, 75, 100, 125, 150 mcg).
- 4For elderly patients, cardiac disease, or severe prolonged hypothyroidism: start at 12.5-25 mcg/day, increasing by 12.5-25 mcg every 6-8 weeks.
- 5For subclinical hypothyroidism: consider whether treatment is indicated (TSH above 10, symptoms, pregnancy intent, anti-TPO positive); if treating, start low (25-50 mcg) and titrate.
- 6Recheck TSH 6-8 weeks after any dose change, as TSH takes 4-6 weeks to reach steady state. Adjust dose aiming for TSH 0.5-2.5 mU/L (standard target) or TSH appropriate for specific indication.
- 7For thyroid cancer, apply TSH suppression targets based on ATA risk stratification, monitoring with TSH every 6-12 months and surveillance imaging per protocol.
Recheck TSH in 6-8 weeks; titrate to TSH 0.5-2.5 mU/L
For a young healthy woman with overt hypothyroidism and no cardiac risk, starting near the calculated full replacement dose is appropriate. A 100 mcg starting dose (nearest available tablet) with TSH rechecked at 6-8 weeks is standard practice.
Do not start at calculated full replacement — risk of precipitating angina or MI
In an elderly patient with known ischaemic heart disease, levothyroxine increases cardiac oxygen demand and can precipitate angina, myocardial infarction, or arrhythmia. Starting at 25 mcg/day with very gradual titration is essential. The TSH target may be set slightly higher (1.0-3.0 mU/L) in frail elderly patients to avoid over-replacement.
Low-risk DTC: TSH low-normal range (not suppressed); recombinant TSH for RAI if needed
For low-risk differentiated thyroid cancer, current ATA guidelines recommend a TSH target in the low-normal range (0.5-2.0 mU/L) rather than suppression, as the oncological benefit of TSH suppression in low-risk DTC is minimal and suppression risks are significant (osteoporosis, atrial fibrillation).
Target FT4 mid-normal: increase levothyroxine until FT4 14-18 pmol/L; TSH value is misleading
In central hypothyroidism, the pituitary cannot raise TSH normally in response to low FT4. TSH will remain deceptively 'normal' or even low. Dose titration must target FT4 in the mid-normal range, with clinical symptom assessment. Over-replacement risk (FT4 above normal) must be avoided as it causes the same downstream risks as primary hyperthyroidism.
Calculating the initial levothyroxine dose for newly diagnosed overt hypothyroidism in primary care
Adjusting levothyroxine dose in pregnancy to maintain first-trimester TSH targets and protect fetal neurodevelopment
Setting TSH suppression targets after thyroidectomy for differentiated thyroid cancer based on ATA risk stratification
Calculating post-bariatric surgery levothyroxine dose adjustments in patients with malabsorptive procedures
Educating patients on the narrow therapeutic window and absorption requirements of levothyroxine
Post-bariatric surgery
Bariatric surgery — particularly Roux-en-Y gastric bypass — reduces the absorptive surface area for levothyroxine in the proximal small bowel. Patients on levothyroxine frequently require substantially higher doses (often 30-50% higher) after gastric bypass. Liquid levothyroxine formulations or intrinsic dissolution preparations may be better absorbed than standard tablets. TSH must be monitored closely in the months after surgery.
Myxoedema coma
Myxoedema coma is a medical emergency requiring immediate intravenous levothyroxine (200-400 mcg IV loading dose followed by 50-100 mcg IV daily) alongside stress-dose IV hydrocortisone (adrenal insufficiency must be excluded before or concurrent with thyroid hormone administration). Gradual rewarming, respiratory support, and electrolyte correction are also required. Oral levothyroxine absorption is unreliable in the setting of reduced gut motility and hypothermia.
Drug interactions affecting levothyroxine requirements
Several medications increase levothyroxine requirements by impairing absorption or increasing metabolism: calcium carbonate, ferrous sulphate, aluminium-containing antacids, proton pump inhibitors, cholestyramine, and sucralfate (impair absorption); rifampicin, carbamazepine, phenytoin, and sertraline (increase hepatic T4 clearance). Patients starting or stopping these medications require closer TSH monitoring.
Congenital hypothyroidism
Neonates with congenital hypothyroidism require prompt treatment with levothyroxine to prevent irreversible intellectual disability. Starting doses are approximately 10-15 mcg/kg/day — substantially higher per kg than adults — because of the critical role of thyroid hormone in early brain development. TSH should normalise within 2 weeks of starting treatment, and FT4 should rise into the upper normal range.
| Indication | TSH Target (mU/L) | Rationale |
|---|---|---|
| Overt primary hypothyroidism — young adult | 0.5 - 2.5 | Symptom resolution; standard target |
| Overt primary hypothyroidism — elderly | 1.0 - 3.0 | Avoid over-replacement; cardiac and bone safety |
| Subclinical hypothyroidism | 0.5 - 2.5 (if treating) | Standard target; individual decision to treat |
| Pregnancy (1st trimester) | 0.1 - 2.5 | Fetal neurodevelopment critical in 1st trimester |
| Pregnancy (2nd-3rd trimester) | 0.2 - 3.0 | Slightly relaxed from first trimester |
| DTC — low risk | 0.5 - 2.0 | Minimal suppression; avoid long-term suppression risks |
| DTC — intermediate risk | 0.1 - 0.5 | Mild suppression for 5 years post-treatment |
| DTC — high risk (active disease) | < 0.1 | Full TSH suppression while active disease present |
| Central hypothyroidism | FT4-guided (not TSH) | TSH unreliable; target FT4 mid-normal range |
Why does levothyroxine need to be taken on an empty stomach?
Levothyroxine absorption from the gastrointestinal tract is 60-80% when taken on an empty stomach but is significantly reduced by calcium (in food, supplements, or antacids), iron, magnesium, aluminium-containing antacids, PPIs (reduce gastric acid needed for dissolution), high-fibre foods, coffee, and soy. Taking levothyroxine 30-60 minutes before breakfast (or 4 hours after the last meal if dosing at night) optimises absorption.
How long does it take for TSH to stabilise after a dose change?
TSH takes approximately 4-6 weeks to fully equilibrate after a change in levothyroxine dose, because TSH is regulated through the hypothalamic-pituitary-thyroid feedback loop, which has a long time constant. Rechecking TSH earlier than 6 weeks after a dose change will typically reflect the transient period rather than the new steady state and may lead to unnecessary dose adjustments.
Should the levothyroxine dose change during pregnancy?
Yes. Levothyroxine requirements increase by approximately 25-50% during pregnancy, often from the first trimester. Women with hypothyroidism who become pregnant should immediately increase their dose by 25-30 mcg (or two extra tablets per week) and have TSH measured urgently. First-trimester TSH targets are typically 0.1-2.5 mU/L. Inadequate replacement is associated with fetal neurodevelopmental impairment and pregnancy loss.
Can generic levothyroxine be substituted for branded formulations?
This is debated. Most patients tolerate generic-branded switching without clinical consequence, as bioequivalence standards require similar peak and total absorption. However, some patients (particularly those with symptoms that correlate closely with their thyroid levels) notice differences between formulations. Stable patients are best maintained on the same manufacturer's formulation rather than being switched at pharmacy dispensing.
What is the evidence for combining levothyroxine with liothyronine (T3)?
Some patients report persistent hypothyroid symptoms despite TSH normalisation on levothyroxine alone. A small subset of patients with the Dio2 polymorphism may have impaired T4-to-T3 conversion in peripheral tissues. Studies of combined T4+T3 therapy have shown modest quality-of-life improvements in some populations. Combination therapy is not routine but may be tried in carefully selected patients who remain symptomatic on adequate levothyroxine monotherapy after other causes have been excluded.
What are the risks of over-treatment with levothyroxine?
Iatrogenic hyperthyroidism (TSH suppression with elevated FT4/FT3) from excessive levothyroxine causes: atrial fibrillation (3-fold increased risk), osteoporosis (particularly in postmenopausal women — increased fracture risk), cardiac hypertrophy, and symptoms of hyperthyroidism (palpitations, heat intolerance, tremor, anxiety). The risks of TSH suppression must be weighed carefully, particularly in thyroid cancer patients where suppression confers modest oncological benefit.
Does body weight or BMI affect levothyroxine requirements?
Yes. Levothyroxine dose correlates with lean body mass (fat has low metabolic activity). For obese patients, ideal body weight rather than actual body weight should be used for dose calculation to avoid systematic overdosing. For underweight patients, actual body weight may be used. Significant weight change (10+ kg) generally warrants a levothyroxine dose review and TSH recheck.
What is liquid levothyroxine and when is it used?
Liquid levothyroxine formulations (solution, soft gel capsules) are used when tablet absorption is impaired — in patients with malabsorption syndromes, Helicobacter pylori gastritis, bariatric surgery (gastric bypass), PPI use, or poor compliance with fasting requirements. Liquid formulations have approximately 20% higher bioavailability than standard tablets for equivalent doses, necessitating dose adjustment when switching.
プロのヒント
When a patient is underdosed despite apparently adequate dose (TSH still elevated), always check: Are they taking the tablet on an empty stomach? Have any new medications been started that could impair absorption (PPIs, calcium, iron)? Have they changed between different tablet formulations? Has body weight changed significantly? These are often more important than simply increasing the dose.
ご存知でしたか？
Levothyroxine (L-thyroxine) was first synthesised in 1927 by Charles Harington and George Barger, about 15 years after Edward Kendall had first isolated thyroxine from thyroid gland extracts in 1914. Despite being one of the simplest hormones structurally — essentially an iodinated amino acid dimer — its correct dosing remains an art requiring careful clinical judgement, not just a formula.
参考文献
- ›Jonklaas J et al. Guidelines for the Treatment of Hypothyroidism (ATA 2014). Thyroid 2014
- ›Haugen BR et al. ATA Management Guidelines for Adult Thyroid Nodules and Differentiated Thyroid Cancer 2015
- ›Alexander EK et al. 2017 ATA Guidelines on Thyroid Disease During Pregnancy and Postpartum. Thyroid 2017
- ›NICE NG145 — Thyroid Disease: Assessment and Management 2019