Levothyroxine Dose Calculator
Levothyroxine Dose Calculator
Estimate a starting levothyroxine dose from body weight and clinical situation, from cautious elderly starts to TSH suppression.
Levothyroxine Dose
Weight × µg/kgBody weight 68 kg, full replacement (1.6 µg/kg)
Formula
- 1.6 µg/kg
- standard full replacement for primary hypothyroidism
- 2.1 µg/kg
- TSH-suppressive dosing after thyroidectomy for differentiated thyroid cancer, per risk-stratified targets
- 1.0 µg/kg
- a cautious starting dose in patients over 65 or with ischaemic heart disease
- 0.5 µg/kg
- a very cautious start, for frail patients or significant cardiac disease
- weight
- ideal or lean body weight is recommended over actual weight in obesity — see the note below
Worked example
Body weight 68 kg, full replacement (1.6 µg/kg)
68 × 1.6 = 108.8, rounded to 109 µg/day
Within 50–125 → typical replacement range
Dose factors by situation
| Situation | Factor (µg/kg/day) | Typical use |
|---|---|---|
| Very cautious start | 0.5 | Frail elderly, significant cardiac disease |
| Elderly / cardiac disease | 1.0 | Start low, titrate slowly |
| Full replacement | 1.6 | Standard adult starting or maintenance dose |
| TSH suppression | 2.1 | Post-thyroidectomy for differentiated thyroid cancer |
Weight, timing and absorption all matter
Full replacement is approximately 1.6 µg/kg of ideal or lean body weight daily, and this distinction matters: using actual body weight in an obese patient systematically overdoses them, because thyroid hormone requirement tracks lean mass and metabolic rate rather than total mass including fat. In patients over 65, or with ischaemic heart disease, the safer approach is to start at 25–50 µg daily and titrate upward every 6–8 weeks, because replacing thyroid hormone too quickly can precipitate angina or arrhythmia in a heart that has adapted to a hypothyroid state.
Whatever the starting dose, recheck TSH no sooner than 6 weeks after any change. The pituitary-thyroid axis takes roughly that long to re-equilibrate, and a TSH checked earlier reflects the old dose more than the new one — checking too early leads to unnecessary further adjustment and a slower path to a stable dose.
Absorption is a frequent, under-recognised source of apparent treatment failure. Levothyroxine should be taken on an empty stomach with water, 30–60 minutes before food, and separated by at least 4 hours from calcium, iron, proton pump inhibitors and soy products, all of which reduce absorption substantially. A patient whose TSH will not settle despite dose increases often turns out to be taking it with breakfast or alongside a calcium supplement.
In pregnancy, requirement rises by approximately 25–30% early in the first trimester, driven by increased TBG and placental metabolism of thyroid hormone, and the dose should be increased as soon as pregnancy is confirmed rather than waiting for a repeat TSH.
Frequently asked questions
What is a typical starting dose of levothyroxine?
About 1.6 µg/kg of ideal or lean body weight daily for full replacement in an otherwise healthy adult. Patients over 65 or with cardiac disease should start lower, around 1.0 µg/kg or 25–50 µg daily, and titrate up.
Which body weight should be used?
Ideal or lean body weight, not actual weight, in patients with obesity — using actual weight overdoses them, since requirement tracks lean mass rather than total mass.
How soon after a dose change should TSH be rechecked?
No sooner than 6 weeks. The pituitary-thyroid axis takes about that long to re-equilibrate after any dose change, so an earlier TSH mostly reflects the previous dose.
How should levothyroxine be taken?
On an empty stomach with water, 30–60 minutes before food, and at least 4 hours apart from calcium, iron, proton pump inhibitors and soy, all of which impair absorption.
Does the dose need to change in pregnancy?
Yes — requirement rises by roughly 25–30% early in pregnancy. Increase the dose as soon as pregnancy is confirmed rather than waiting for the next scheduled TSH.
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References
- Jonklaas J et al. Guidelines for the treatment of hypothyroidism. American Thyroid Association Task Force. Thyroid. 2014;24(12):1670–751.
- Haugen BR et al. 2015 American Thyroid Association Management Guidelines for adult patients with thyroid nodules and differentiated thyroid cancer. Thyroid. 2016;26(1):1–133.