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Printed on 6/29/2026
For informational purposes only. This is not medical advice.
This calculator provides an initial levothyroxine dose estimate using weight-based dosing guidelines. Full replacement averages 1.6 mcg/kg/day, TSH suppression requires ~2.0 mcg/kg/day, and elderly or subclinical hypothyroidism starts lower. Dose is titrated based on TSH levels — monitor therapy with [TSH Interpreter](/tools/tsh-interpreter). Dosing is weight-based — for accurate weight assessment use [BMI Calculator](/tools/bmi-calculator) and [Ideal Weight Calculator](/tools/ideal-weight). In elderly patients, unintentional overtreatment increases atrial fibrillation risk — assess with [CHA2DS2-VASc Score](/tools/cha2ds2-vasc).
Formula: Dose = Weight × factor. Replacement: 1.6 mcg/kg. Suppression: 2.0 mcg/kg. Subclinical/elderly: 1.0 mcg/kg.
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Input body weight in kilograms or pounds. The standard full replacement dose for adults is approximately 1.6 mcg/kg/day based on pharmacokinetic studies establishing the average T4 production rate. For obese patients, ideal body weight (IBW) is used rather than actual weight to avoid supratherapeutic dosing.
Choose from full replacement (post-thyroidectomy or autoimmune hypothyroidism), TSH suppression (differentiated thyroid cancer, targeting TSH below 0.1 mIU/L requiring ~2.0 mcg/kg/day), or subclinical/elderly dosing (conservative start at ~1.0 mcg/kg/day or 12.5–25 mcg/day for cardiac patients to minimize risk of atrial fibrillation and osteoporosis).
The suggested starting dose is displayed and rounded to the nearest available tablet strength (25, 50, 75, 88, 100, 112, 125, 137, 150, 175, 200 mcg). This is a starting dose only — TSH should be rechecked 6–8 weeks after initiation to guide titration toward your individual target range.
Endocrinologists, surgeons, primary care
After total thyroidectomy, the patient produces zero endogenous T4 and requires full replacement. The 1.6 mcg/kg/day estimate provides a practical starting point, though many patients stabilize at slightly different doses depending on residual tissue and absorption factors.
Nuclear medicine, endocrinologists
Following radioactive iodine ablation for Graves' disease or differentiated thyroid cancer, residual thyroid tissue is destroyed over weeks to months. Starting levothyroxine at an appropriate replacement dose after thyroid function declines prevents symptomatic hypothyroidism during this transition period.
Primary care, endocrinologists
When autoimmune thyroiditis leads to overt hypothyroidism (TSH above 10 mIU/L or symptomatic TSH elevation), levothyroxine initiation at weight-based dosing normalizes TSH in the majority of patients within 6–12 weeks.
Geriatricians, cardiologists, internists
Elderly patients and those with coronary artery disease or atrial fibrillation require a conservative initiation strategy. Start with 12.5–25 mcg/day and increase by 12.5–25 mcg every 4–8 weeks to avoid precipitating angina, arrhythmia, or heart failure exacerbation.
Obstetricians, endocrinologists, maternal-fetal medicine
Levothyroxine requirements increase 25–50% during pregnancy, typically beginning by weeks 4–6 of gestation. Pre-conception dose adjustments and early first-trimester TSH testing (target TSH below 2.5 mIU/L) are critical for fetal neurological development.
Primary care, endocrinologists
Subclinical hypothyroidism (TSH 4.5–10 mIU/L, normal free T4) has selective treatment indications: pregnancy or planning pregnancy, TSH above 10 mIU/L, symptomatic patients, or patients under 65. Conservative dosing starting at 25–50 mcg is typical for this indication.
Levothyroxine absorption is maximized when taken at least 30–60 minutes before breakfast on an empty stomach. Food, coffee, and dairy can reduce absorption by 20–40%. Some patients prefer bedtime dosing (3 hours after last meal) which also achieves consistent absorption and may slightly improve TSH levels.
Calcium carbonate, calcium citrate, iron supplements, and proton pump inhibitors (PPIs) all significantly impair levothyroxine absorption and should be separated by at least 4 hours. This is one of the most common reasons for unexpectedly high TSH despite an apparently adequate dose.
For patients with BMI above 30, using actual body weight systematically overestimates levothyroxine requirements. Use ideal body weight (IBW) for dose calculation: IBW (kg) = 50 + 2.3 × (height in inches − 60) for males; 45.5 + 2.3 × (height in inches − 60) for females.
TSH has a half-life of approximately one week, but levothyroxine and its effects on pituitary TSH secretion take 6–8 weeks to reach a new steady state after any dose change. Do not check TSH sooner — values before this point will be unreliable and may lead to unnecessary further adjustments.
For primary hypothyroidism replacement, most guidelines recommend targeting TSH 0.5–2.5 mIU/L. Some clinicians target 1.0–2.0 mIU/L for patients with persistent symptoms. For differentiated thyroid cancer, TSH suppression targets are risk-stratified: TSH below 0.1 for high-risk, 0.1–0.5 for intermediate, and 0.5–2.0 for low-risk long-term surveillance.
Different branded and generic formulations of levothyroxine have demonstrated variable bioavailability. Switching between brands (Synthroid, Levoxyl, Tirosint, generics) can cause TSH drift. The FDA classifies levothyroxine as a narrow therapeutic index drug. When possible, maintain the patient on one consistent formulation and recheck TSH if any switch is made.
In women with known hypothyroidism who become pregnant, levothyroxine dose should be empirically increased by approximately 30% (some practitioners add 2 extra tablets per week) immediately upon confirmed pregnancy. TSH should be rechecked every 4 weeks during the first trimester — more frequently than usual — due to rapidly changing requirements.
Initiating levothyroxine in patients with ischemic heart disease, heart failure, or atrial fibrillation requires extreme caution. Begin with 12.5–25 mcg/day and increase by 12.5–25 mcg every 4–8 weeks under cardiology co-management. Abrupt full replacement dosing in long-standing severe hypothyroidism can precipitate myocardial infarction or arrhythmia.
Unlike many medications, because levothyroxine has a long half-life (7 days), missing one or even two doses does not cause acute toxicity. If you miss a dose, take it as soon as you remember — even if it means doubling up on the same day for two missed doses. Consistent daily use is far more important than timing accuracy for any single missed dose.
With aging, T4 clearance decreases, meaning elderly patients often achieve target TSH at lower doses (sometimes 25–50% less than younger adults). Over-replacement in older patients is a common cause of iatrogenic atrial fibrillation and accelerated bone loss. Regular monitoring of TSH every 6–12 months is essential in this population.
Levothyroxine dosing is based on the ATA Guidelines for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum (Thyroid 2017). Full replacement dose of 1.6 mcg/kg/day is established from pharmacokinetic studies of T4 production and clearance rates. Cardiac and elderly starting dose recommendations follow endocrinology practice consensus from the ATA/AACE/TES joint guidelines. Absorption interactions are reviewed in Liwanpo & Hershman (Thyroid 2009). Biotin interference with TSH immunoassays documented by Piketty et al. (Eur J Endocrinol 2017).
Your estimated levothyroxine starting dose is calculated based on your body weight and clinical indication. For full thyroid hormone replacement in primary hypothyroidism, the standard dose is approximately 1.6 mcg/kg/day, which provides adequate T4 to normalize TSH in most adults. For TSH suppression therapy (e.g., after thyroid cancer), higher doses near 2.0 mcg/kg/day are typically required to achieve a TSH below 0.1 mIU/L. For subclinical hypothyroidism or elderly patients, a conservative starting dose of approximately 1.0 mcg/kg/day or even 12.5–25 mcg/day is recommended to avoid cardiac complications.
This result is a starting point only. Levothyroxine dosing is highly individualized, and your actual requirement depends on residual thyroid function, body composition, absorption factors, and concomitant medications. TSH should be rechecked 6–8 weeks after initiation or any dose adjustment to guide titration.
Use this calculator when initiating levothyroxine therapy for a newly diagnosed hypothyroid patient, whether from autoimmune thyroiditis (Hashimoto's), post-thyroidectomy, post-radioactive iodine ablation, or subclinical hypothyroidism. It is also helpful when estimating a TSH suppression dose for differentiated thyroid cancer patients.
This tool is intended for adult dosing. Pediatric thyroid hormone dosing uses different weight-based calculations. It is most useful in the outpatient setting when starting therapy, though inpatient clinicians may also reference it when converting from IV levothyroxine (which is approximately 75% of the oral dose).
This calculator provides a weight-based estimate and does not account for factors that significantly alter levothyroxine requirements, including malabsorption syndromes (celiac disease, bariatric surgery), concurrent medications that reduce absorption (calcium, iron, PPIs, cholestyramine), pregnancy (which increases requirements by 25–50%), and age-related changes in metabolism.
The calculator assumes a standard oral bioavailability of approximately 70–80%. Actual doses are typically rounded to the nearest available tablet strength (25, 50, 75, 88, 100, 112, 125, 137, 150, 175, 200 mcg). Overreplacement carries risks of atrial fibrillation and osteoporosis, particularly in elderly patients, so conservative initiation with gradual titration is preferred in high-risk populations.
For related assessments, see TSH Interpreter and HbA1c Converter.
Disclaimer: This tool is for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider with questions about your health.
April 21, 2026 · trust-baseline
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