Estimate a weight-based starting dose of levothyroxine (Synthroid) for hypothyroidism, post-thyroidectomy replacement, or TSH suppression. Results are educational only — always confirm with your prescriber.
Levothyroxine (T4) is the most commonly prescribed thyroid hormone replacement. The body converts T4 to the active form T3 (triiodothyronine) in peripheral tissues. Weight-based dosing provides a starting estimate — final dose is always titrated to TSH and symptoms over weeks to months.
Why weight matters: Lean body mass (not total weight) drives thyroid hormone distribution. Obese patients are typically dosed on ideal or adjusted body weight. Pregnant women require 25–50% higher doses due to increased thyroxine-binding globulin.
Titration schedule: TSH is rechecked 6–8 weeks after any dose change. Dose adjustments are typically 12.5–25 mcg increments. Full steady-state takes 4–6 weeks after each change.
What Affects Absorption
Empty stomach
Levothyroxine must be taken 30–60 minutes before food. Food reduces absorption by 20–40%.
Calcium supplements
Calcium chelates levothyroxine. Separate by at least 4 hours.
Iron supplements
Iron significantly reduces absorption. Separate by at least 4 hours.
Coffee
Coffee — including espresso — reduces absorption when taken within 60 minutes of dosing. Use water only at time of dose.
Antacids / PPIs
Calcium-containing antacids and proton pump inhibitors reduce absorption. Consistency in timing relative to these is key.
Frequently Asked Questions
How is levothyroxine dose determined?
Starting dose is typically 1.6 mcg per kg of body weight for hypothyroidism. Post-thyroidectomy requires 1.8 mcg/kg and TSH suppression requires 2.0 mcg/kg. The dose is then adjusted based on TSH levels after 6-8 weeks.
Why do I need to take levothyroxine on an empty stomach?
Food — especially calcium, iron, soy, and coffee — significantly reduces absorption. Take it 30-60 minutes before breakfast or at bedtime (3+ hours after last meal) for consistent absorption.
How long until levothyroxine works?
TSH takes 6-8 weeks to stabilize after starting or changing dose. You may feel symptom improvement in 2-4 weeks, but do not adjust the dose before 6 weeks of lab confirmation.
Can I take levothyroxine with other medications?
Many drugs interfere with absorption. Separate calcium and iron supplements by 4 hours. PPIs, antacids, and cholestyramine also reduce absorption. Take levothyroxine first, everything else later.
Why is the starting dose lower for elderly patients?
Older adults and those with cardiac history risk angina, arrhythmia, or heart failure from sudden thyroid hormone increases. Starting at 25-50 mcg and increasing by 12.5-25 mcg every 6-8 weeks is safer.
Track your thyroid labs and medication effectiveness
Upload TSH, Free T4, and Free T3 results to Vitalix. Track dose changes over time, correlate symptoms with labs, and get AI-powered insights from your thyroid specialist.
Medical Disclaimer: Do not start or adjust thyroid medication without consulting your doctor. This calculator provides educational estimates only and is not a substitute for professional medical advice, diagnosis, or treatment. Individual dosing depends on age, weight, cardiac history, comorbidities, concomitant medications, and laboratory values interpreted by a licensed clinician.