π Levothyroxine Dose Calculator
Weight-based replacement estimate. Your doctor sets the dose.
For informational purposes only, not medical advice. Dosing decisions must be confirmed with your doctor or pharmacist. Consult a healthcare professional.
Full thyroid hormone replacement averages 1.6 micrograms per kilogram per day. A healthy adult starting levothyroxine for overt hypothyroidism typically begins near that full replacement dose, then TSH testing 6 to 8 weeks later fine-tunes it. This calculator gives the weight-based estimate and the standard adjustments for age, heart disease, and pregnancy.
Three groups break the standard rule. Adults over 60 and anyone with heart disease start at 12.5 to 25 mcg daily and titrate slowly, because thyroid hormone raises heart rate and oxygen demand. Pregnancy raises requirements by roughly 25 to 50%, so doses go up, not down, with close TSH monitoring.
How to Use This Calculator
Enter weight in kilograms and pick the patient group. Read the full replacement estimate and the suggested starting approach. Expect the real prescription to round to available tablet strengths (25 through 150+ mcg). Take it on an empty stomach, 30 to 60 minutes before breakfast, and recheck TSH 6 to 8 weeks after any change.
Enter weight in kg and pick the patient group.
Read the full replacement estimate (1.6 mcg/kg/day).
Note the starting guidance: full dose for healthy adults, low-and-slow for elderly/cardiac, higher for pregnancy.
Confirm the actual prescription and timing with your doctor.
The 1.6 mcg/kg Rule
Elderly/cardiac start = 12.5-25 mcg/day
Pregnancy = pre-pregnancy dose x 1.25 to 1.5
The 1.6 mcg/kg/day figure comes from studies of athyreotic patients (no thyroid function at all) whose TSH normalized at that average dose. Real requirements scatter around it: lean body mass predicts better than total weight, so very obese patients often need less per kilogram, and malabsorption, soy, coffee, calcium, and iron all reduce absorption. Levothyroxine has a 7-day half-life, which is why missed doses are forgiving and why TSH takes 6 to 8 weeks to reflect a change. The elderly start-low rule exists because even mild overtreatment can trigger angina or atrial fibrillation in a vulnerable heart.
Worked Example
A 70 kg healthy adult: full replacement estimates to 112 mcg/day, so the prescription usually lands on the nearest tablet, often 112 mcg. For an older adult with heart disease the same weight suggests 112 mcg/day instead, titrated upward slowly. In pregnancy, needs rise: Full replacement estimate
Reading Your Estimate
The full replacement number is the destination; the start dose is the on-ramp. Healthy young adults often start at or near full replacement. Older or cardiac patients start at 12.5 to 25 mcg and climb in small steps every 6 to 8 weeks. Pregnancy is the exception that goes the other way: most women need about 25 to 50% more, often implemented as two extra tablets per week, with TSH checked each trimester. Symptoms lag labs by weeks, so dose changes are driven by TSH, not by how you feel on day ten.
Thyroid Dosing Needs a Doctor
Do not start, stop, or adjust levothyroxine on a calculator result. Overtreatment causes atrial fibrillation and bone loss over time; undertreatment in pregnancy harms fetal brain development. Biotin supplements famously distort thyroid labs, so stop biotin a few days before testing and tell your doctor you take it. This tool is for informational purposes only, not medical advice. Dosing decisions must be confirmed with your doctor or pharmacist. Consult a healthcare professional.
Related tools: BMI calculator and iron deficiency checker.
For informational purposes only, not medical advice. Dosing decisions must be confirmed with your doctor or pharmacist. Consult a healthcare professional.
Frequently Asked Questions
What is the standard levothyroxine dose?
About 1.6 mcg/kg/day for full replacement in healthy adults, then adjusted by TSH. Most adults land between 75 and 150 mcg daily.
Why start low in older adults?
Thyroid hormone increases heart rate and oxygen demand. In older or cardiac patients, starting at 12.5 to 25 mcg avoids triggering angina or arrhythmia.
How should I take levothyroxine?
On an empty stomach with water, 30 to 60 minutes before food or 3 to 4 hours after the last meal. Calcium, iron, coffee, and soy too close to the dose reduce absorption.
How long until TSH reflects a dose change?
Six to 8 weeks, because of levothyroxine's week-long half-life. Testing sooner just measures the transition.
What if I miss a dose?
Take it when you remember unless it is almost time for the next one. The long half-life makes occasional misses low-drama; do not double up routinely.
Does weight change my dose?
Yes. Significant weight gain or loss usually requires rechecking TSH and adjusting. Pregnancy, which is coming in the FAQs of life, raises needs 25 to 50%.
Can biotin affect thyroid labs?
Yes, high-dose biotin can falsely alter TSH and free T4 results. Stop it 2 to 3 days before labs and tell your doctor.
Is generic levothyroxine as good as Synthroid?
Generics are bioequivalent by FDA standards, but thyroid patients are sensitive to small changes, so stick with one manufacturer and recheck TSH if the pharmacy switches it.