Professional reference. Individualise doses, and follow local protocols and endocrinology advice, particularly for cardiac patients, the elderly and myxoedema coma.
Quick reference
- PO → IV: IV dose ≈ 0.5–0.75 × oral dose, once daily.
- IV → PO: resume the previous oral dose (or IV dose ÷ 0.75 if no oral history).
- Short nil-by-mouth periods (a few days): IV replacement is often not needed; doses can be held and resumed.
Why the IV dose is lower
Oral levothyroxine is absorbed from the small intestine, with bioavailability of roughly 60–80% in healthy adults and lower with food, coffee, calcium, iron, PPIs or gut disease. An IV dose is 100% available, so less is needed for the same effect. US prescribing information for levothyroxine injection recommends starting IV at about 50–75% of the oral dose, with ongoing clinical and laboratory monitoring. The American Thyroid Association guideline gives similar advice.
Conversion table
| Oral dose (mcg/day) |
IV at 50% (mcg/day) |
IV at 75% (mcg/day) |
| 25 |
12.5 |
19 (≈ 20) |
| 50 |
25 |
37.5 |
| 75 |
37.5 |
56 (≈ 55–60) |
| 100 |
50 |
75 |
| 125 |
62.5 |
94 (≈ 95) |
| 150 |
75 |
112.5 |
| 175 |
87.5 |
131 |
| 200 |
100 |
150 |
Round to a practical volume for the vial strength in use (commonly 100, 200 or 500 microgram vials reconstituted with sodium chloride 0.9%). Choose the lower end for older patients and those with heart disease, and the higher end where oral absorption was known to be good.
Worked example
A 68-year-old on levothyroxine 125 micrograms orally daily is expected to be nil by mouth for 10 days after bowel surgery. Using 0.6 as a cautious factor: 125 × 0.6 = 75 micrograms IV once daily. Check thyroid function after about 1–2 weeks if IV therapy continues, and watch for tachycardia or angina. When eating again, restart 125 micrograms orally.
When is IV levothyroxine needed?
- Short interruptions (up to about 5–7 days): usually no IV needed, given the long half-life (about 7 days in euthyroid adults).
- Prolonged inability to take oral or enteral medicines: start IV at the converted dose.
- Enteral feeding: often the tablet can be given via the tube with a feed break; absorption may fall, so monitor TSH.
- Myxoedema coma: an emergency with separate loading-dose regimens (typically 200–400 micrograms IV, then daily doses), managed with endocrinology and critical care, with steroid cover until adrenal insufficiency is excluded.
Other conversions and monitoring notes are in the clinical pharmacy notes.
Frequently asked questions
What is the levothyroxine IV to PO conversion?
Going from IV to oral, divide the IV dose by 0.75 (or by 0.5–0.75), or simply resume the patient’s previous oral dose if known. Recheck TSH 6–8 weeks after the switch.
Can IV levothyroxine be given as a bolus?
Yes; it is usually given as a slow IV injection after reconstitution, as per the product information. Check compatibility before giving it via a line with other infusions.
¿Cuál es la equivalencia de levotiroxina oral a IV?
La dosis intravenosa suele ser del 50–75% de la dosis oral, una vez al día. Por ejemplo, 100 mcg por vía oral equivalen a unos 50–75 mcg por vía intravenosa.
Sources and further reading
CanforaMedicine Guide
