The Levothyroxine IV ↔ Oral Conversion calculator helps clinicians estimate an equivalent dose when switching a patient between intravenous (IV) and oral (PO) routes of administration. This conversion is crucial for maintaining therapeutic continuity in patients who are temporarily unable to take oral medications, such as those with NPO (nothing by mouth) status.

The tool is based on the widely accepted conversion ratio that accounts for the differences in bioavailability between the two formulations. It is intended for informational purposes and should not replace professional clinical judgment.

The calculator provides one primary output:

• Equivalent Dose (mcg): This is the calculated dose in micrograms for the target route of administration (either oral or IV). The result is presented alongside the calculation steps, showing how the input dose was adjusted using the standard conversion ratio.

The output also includes a recommendation to round the calculated value to the nearest commercially available tablet or vial strength, which requires clinical discretion.

Follow these simple steps to determine the equivalent levothyroxine dose:

1. Select Conversion Direction: Choose whether you are converting from an IV dose to an Oral dose or from an Oral dose to an IV dose.
2. Enter Current Dose: Input the patient's current, stable dose in micrograms (mcg) for the selected starting route.
3. Calculate and Review: Click the "Calculate" button. The tool will display the equivalent dose for the new route of administration. Always review the calculation and apply clinical judgment.

The standard conversion between intravenous and oral levothyroxine is based on a ratio where the IV dose is 50% of the PO dose (a 0.5:1 IV:PO ratio). This accounts for the incomplete and variable oral bioavailability of levothyroxine, which is typically estimated to be 60-80%.

For example, a patient stable on 150 mcg of oral levothyroxine would require an equivalent IV dose of approximately 75 mcg (150 mcg * 0.5).

This conversion is primarily for maintenance dosing in patients with a previously established therapeutic requirement. It does not apply to loading doses for conditions like myxedema coma, which requires a separate, high-dose protocol.

Switching from oral to IV administration is typically reserved for patients who cannot tolerate or absorb oral medications. Common scenarios include:

• Pre- and post-operative NPO status.
• Critical illness with impaired gastrointestinal function or absorption.
• Severe vomiting or malabsorption syndromes.

When switching back from IV to oral, the patient should be able to tolerate and absorb oral intake. In either case, it is good practice to monitor thyroid function tests (e.g., TSH) after the switch to ensure the new dose maintains euthyroidism, especially if the change in route is prolonged.

Levothyroxine (T4) has a very long half-life of approximately 7 days. Because of this, missing a single dose or a brief delay in switching from oral to IV administration is often not clinically significant for a euthyroid patient. Clinical judgment should guide decisions regarding missed doses, but the extended half-life provides a degree of flexibility in administration.

This tool is for educational purposes only and is not a substitute for clinical judgment.

• Verify All Calculations: Independently verify all dose calculations before medication administration.
• Individualize Dosing: Patient-specific factors, including age, weight, cardiovascular status, and severity of illness, may necessitate dose adjustments beyond this standard conversion.
• Round Appropriately: Always round the calculated dose to the nearest available commercial tablet or vial strength. Consult institutional protocols and available formulations.
• Not for Myxedema Coma: This calculator is not intended for calculating loading or maintenance doses for myxedema coma, which requires a distinct treatment protocol.

What is the standard conversion ratio for IV to oral levothyroxine?
The standard conversion is that the intravenous (IV) dose is 50% of the oral (PO) dose. This is an IV:PO ratio of 0.5:1.

Why is the IV dose of levothyroxine lower than the oral dose?
The IV dose is lower because it has 100% bioavailability, meaning the entire dose enters the bloodstream. The oral form has incomplete bioavailability (around 60-80%), so a higher dose is needed to achieve the same effect.

When should I switch a patient from oral to IV levothyroxine?
A switch is indicated when a patient on a stable dose is unable to take oral medications, for example, due to being NPO for surgery, critical illness, or severe gastrointestinal issues preventing absorption.

Does this calculator apply to myxedema coma?
No. Myxedema coma is a life-threatening emergency requiring a much higher IV loading dose and a specific treatment protocol. This calculator is only for standard maintenance dose conversion.

How should I round the calculated levothyroxine dose?
You should round to the nearest commercially available tablet strength (for oral) or vial strength (for IV). Clinical judgment is essential in deciding whether to round up or down.

How often should TSH be monitored after switching from oral to IV levothyroxine?
While the long half-life provides some buffer, it is prudent to check a TSH level after the switch, especially if IV therapy is expected to continue for an extended period, to ensure the dose is therapeutic.

Is this conversion the same for all brands of levothyroxine?
Yes, the conversion principle applies to the active ingredient, levothyroxine sodium, regardless of brand. However, consistency with one brand is generally recommended for patients when possible.

Can I use this calculator for liothyronine (T3)?
No. This calculator is designed exclusively for levothyroxine (T4). Liothyronine has different pharmacokinetics and dosing conversions.

• Jonklaas, J., Bianco, A. C., Bauer, A. J., et al. (2014). Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association task force on thyroid hormone replacement. Thyroid, 24(12), 1670-1751. View on PubMed
• Levothyroxine Sodium for Injection [package insert]. Lake Forest, IL: Hospira, Inc.; 2017. View PDF
• Levothyroxine. In: Lexicomp® Online. Wolters Kluwer Health. Accessed on various dates. (Requires subscription)
• Levothyroxine. In: IBM Micromedex® RED BOOK®. Accessed on various dates. (Requires subscription)