MDCalc

Weight-based Levothyroxine Dose Calculator for Hypothyroidism in Adults

Determines a weight-based levothyroxine dose for treatment of primary hypothyroidism.

Enter the patient’s body weight (kg or lbs) to calculate the full oral thyroid replacement dose of levothyroxine for the treatment of hypothyroidism.

Result:

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Advice
  • Prescribing guidance:
    • Levothyroxine tablets are available in standard doses: 25, 50, 75, 88, 100, 112, 125, 137, 150, 175, 200, and 300 mcg.
    • Round the calculated dose by 10–15 mcg to the closest available tablet strength.
    • If a nonstandard dose is desired, clinicians may use pill combinations, tablet splitting, or dosing that varies by day of the week. 
    • Levothyroxine’s long half-life (approximately 7 days) ensures stable thyroid hormone levels despite day-to-day variations; for example, an extra pill 1–2 times per week can be employed to increase a dose.
    • The choice between brand name and generic levothyroxine is guided by availability, cost, and patient preference, as they are equally efficacious.
  • Patient instructions:
    • Take levothyroxine on an empty stomach with water, ideally first thing in the morning.
    • Wait 30–60 minutes before eating or taking other medications.
    • Separate calcium or iron supplements (including multivitamins) from levothyroxine by at least 4 hours.
    • If evening administration is preferred, take levothyroxine 3–4 hours after the last meal or snack.
    • If a dose is missed, it is generally safe to make up for the missed dose by taking double the levothyroxine the following day.
  • Other considerations: Requirements that significantly exceed the weight-based dose or >200–300 mcg/day suggest possible malabsorption or medication nonadherence and warrant further evaluation.
Management
  • Prescribe the closest oral levothyroxine dose available, or employ pill combinations, splitting, or alternate-day dosing to achieve a dose close to the calculated value.
  • If intravenous levothyroxine therapy is needed, use ~30% lower doses than oral to account for increased intravenous bioavailability. 
  • For ongoing management, monitor TSH and clinical response every 4–8 weeks, adjusting the dose by 12.5–25 mcg as needed. 
  • Consult an endocrinologist for complex or refractory cases, or for further questions or concerns regarding thyroid disease management.
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