Levothyroxine oral to IV conversion calculator
Estimates the IV levothyroxine dose that corresponds to a daily oral dose, or the oral dose after IV therapy. The result is a starting estimate; labels allow 50% to 75% of the oral dose by vein.
- Conversions
- Label
- Runs on your device
Short answer
The IV levothyroxine dose is estimated as the daily oral dose × 0.75, the ceiling in the UK product licence; labels allow 50% to 75% of the oral dose by vein. The reverse divides the IV dose by 0.75.
Levothyroxine oral–IV conversion
Runs on your device. Nothing you enter is saved or sent.
Estimate only. Interpret it with the patient's clinical picture and the product label.
Sources
3
- Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association task force on thyroid hormone replacement. Thyroid. 2014;24(12):1670–1751. DOI 10.1089/thy.2014.0028 PubMed 25266247
- Levothyroxine SERB 200 micrograms/mL solution for injection/infusion: Summary of Product Characteristics (UK, eMC product 15068). Electronic Medicines Compendium (eMC). Not indexed in PubMed
- Levothyroxine sodium for injection: US prescribing information (FDA label). US Food and Drug Administration, DailyMed. Not indexed in PubMed
Formula version: Levothyroxine oral–IV (UK SmPC · ATA 2014) · MediHelp calc v1.3.1
Formula and sources checked on 2026-11-02
When this estimate is not valid
5
- The 75% factor is the upper limit in the UK SmPC (Levothyroxine SERB) and reflects oral bioavailability of roughly 50% to 80%; US labels state the relative bioavailability has not been established. The result is a starting estimate.
- TSH and free T4 are usually checked about 4 to 6 weeks after a route change, and the dose is adjusted to the same TSH target as before.
- Oral absorption varies with food, coffee, proton pump inhibitors, calcium, iron, bile acid sequestrants and gut disease, so the patient's oral dose may itself be more than they absorb.
- Myxoedema coma uses an IV loading dose (commonly 200 to 400 mcg) followed by maintenance; that regimen comes from guidelines, not from this conversion.
- Does not convert liothyronine (T3) or combination products.
Estimate only. Interpret it with the patient's clinical picture and the product label.
About this calculator
- What it is
- Estimates the IV levothyroxine dose that matches an oral dose, or the oral dose after IV therapy.
- Why it is used
- Patients who are nil by mouth or on long-term enteral feeds sometimes need IV levothyroxine, and the IV dose is lower because oral absorption is incomplete.
- How to use it
- Multiplies the oral dose by 0.75 (the ceiling in the UK product licence), and shows the 50% to 75% range that labels allow; the reverse divides by the same factor.
- How to read the result
- A starting estimate only: TSH after 4 to 6 weeks confirms it, and short interruptions of oral therapy rarely need IV cover.
How it is calculated
IV dose ≈ oral dose × 0.75 (range 0.50 to 0.75); oral dose ≈ IV dose ÷ 0.75
The IV dose is the oral dose multiplied by 0.75, the ceiling in the UK product licence. The reverse divides the IV dose by 0.75.
The range line shows the lower end as well, so the 75% is seen as a ceiling: for oral to IV it runs from 50% to 75% of the oral dose; for IV to oral from the IV dose ÷ 0.75 to the IV dose ÷ 0.50.
Enter the daily dose in micrograms (mcg), not milligrams. The result is not rounded to tablet strengths or vial sizes.
Notes appear for an oral-equivalent dose above 300 mcg a day, for a dose below 12.5 mcg, and always when converting oral to IV (short interruptions of oral therapy rarely need IV cover).
Limits: a dose from 5 to 1000 mcg. The calculator shows each step of the arithmetic.
Worked example
100 mcg of oral levothyroxine daily converted to IV:
100 × 0.75 = 75.0 mcg100 × 0.50 = 50.0; 100 × 0.75 = 75.0 mcg
100 mcg of oral levothyroxine is approximately equivalent to 75.0 mcg IV daily, within a range of 50.0 to 75.0 mcg (a starting estimate).
75 mcg of IV levothyroxine daily converted to oral:
75 ÷ 0.75 = 100.0 mcg75 ÷ 0.75 = 100.0; 75 ÷ 0.50 = 150.0 mcg
75 mcg of IV levothyroxine is approximately equivalent to 100.0 mcg orally daily, within a range of 100.0 to 150.0 mcg (a starting estimate).
Questions
Why is the IV dose lower than the oral dose?
Oral levothyroxine is only partly absorbed (roughly 50% to 80%), while the IV dose reaches the circulation completely.
Is 75% exact?
No. It is the upper limit in the UK SmPC; US labels state that the relative bioavailability has not been established. The result is a starting estimate.
How is the effect checked after a route change?
TSH and free T4 are usually checked about 4 to 6 weeks after a route change, and the dose is adjusted to the same TSH target as before.
Does it cover myxoedema coma?
No. Myxoedema coma uses an IV loading dose followed by maintenance, taken from guidelines and not from this conversion.
Does it convert liothyronine (T3)?
No. It does not convert liothyronine or combination products.
Use the calculators in the MediHelp app
Estimate only. Interpret it with the patient's clinical picture and the product label.
