The Thyroid Hormone Conversion Calculator (T4 ↔ T3) is an educational resource for healthcare professionals to estimate dose equivalency when converting a patient from levothyroxine (T4) monotherapy to liothyronine (T3) monotherapy, or vice versa. This conversion is based on the differing biological potencies of the two thyroid hormones.

• Equivalent Dose: The primary output is the calculated dose in micrograms (mcg) of the target hormone (e.g., T3) that is estimated to be equivalent to the starting dose of the original hormone (e.g., T4).
• Conversion Summary: The tool displays the starting hormone and dose, the target hormone and calculated dose, and the potency ratio used for the calculation (e.g., 3:1).
• Clinical Note: A brief disclaimer is provided to remind users that this is an estimation and clinical judgment, along with laboratory monitoring, is essential for actual patient dosing.

1. Select Conversion Direction: Choose whether you are converting from T4 to T3 or from T3 to T4.
2. Enter Starting Dose: Input the patient’s current daily dose of the starting hormone in micrograms (mcg).
3. Review the Calculation: The calculator will automatically provide the estimated equivalent daily dose of the target hormone based on a standard 3:1 potency ratio.

Levothyroxine (T4) is the standard-of-care for hypothyroidism treatment due to its long half-life (~7 days), which allows for stable serum levels with once-daily administration. T4 is a prohormone that is peripherally deiodinated to the active hormone, T3.

Liothyronine (T3) is approximately 3 to 4 times more potent than T4. It has a much shorter half-life (~1 day), leading to rapid peaks and troughs in blood levels. To maintain stability, T3 administration often requires splitting the total daily dose into two or three smaller doses. The rapid action of T3 increases the risk of thyrotoxic symptoms, particularly in sensitive populations.

Switching between T4 and T3 monotherapy is uncommon and reserved for specific clinical scenarios. It is not a direct dose-for-dose exchange. The process requires careful planning and monitoring:

• Initial Dose Estimation: Use the calculated equivalent dose as a starting point, not a final prescription. Clinicians may choose to start with a slightly lower dose and titrate up.
• Administration Schedule: When switching from T4 to T3, the dosing schedule must change from once-daily to multiple times per day to avoid large fluctuations in serum T3 levels.
• Laboratory Monitoring: After switching, serum TSH should be re-evaluated in 4-6 weeks to assess the adequacy of the new dose and make necessary adjustments. Monitoring for clinical signs and symptoms of hyper- or hypothyroidism is crucial during the transition period.

• Levothyroxine (T4): Due to its long half-life, a single missed dose is unlikely to have a significant clinical impact. The patient can generally take the missed dose as soon as they remember. If they remember the next day, they should not double the dose and should simply resume their normal dosing schedule.
• Liothyronine (T3): A missed dose is more likely to cause transient symptoms due to the short half-life. The patient should take the dose as soon as they remember, but if it is close to the time of their next scheduled dose, they should skip the missed one and continue their regular schedule. Doubling up on doses should be avoided. Patients should be advised to contact their healthcare provider for specific instructions.

Thyroid hormones have a narrow therapeutic index. Over- or under-treatment can have significant health consequences.

• Risk of Thyrotoxicosis with T3: The high potency and rapid onset of T3 increase the risk of iatrogenic hyperthyroidism. Symptoms include palpitations, tachycardia, arrhythmias, nervousness, tremor, and increased risk of angina or myocardial infarction in patients with underlying cardiovascular disease.
• Special Populations: Caution is essential in elderly patients, pregnant women, and individuals with cardiovascular conditions, as they are more susceptible to adverse effects.
• Not for Weight Loss: Thyroid hormones, alone or with other agents, should not be used for the treatment of obesity or for weight loss. Large doses can cause serious or even life-threatening manifestations of toxicity.

What potency ratio does this calculator use for T4 to T3 conversion?
This tool uses a 3:1 ratio, meaning 3 mcg of T4 is considered roughly equivalent to 1 mcg of T3. Some sources cite a ratio up to 4:1, but 3:1 is a common and conservative starting point for clinical estimation.

Why is T3 considered more potent than T4?
T3 is the biologically active form of thyroid hormone that binds to thyroid hormone receptors in cells to exert its metabolic effects. T4 must first be converted to T3 to become active, making T3 more potent on a microgram-for-microgram basis.

Can I use this calculator to determine doses for T4/T3 combination therapy?
No. This calculator is designed for converting between T4 and T3 monotherapies only. Combination therapy dosing is complex and requires specialized clinical judgment beyond the scope of this tool.

How accurate is the calculated equivalent dose?
The result is a guideline estimation. Individual patient factors like absorption, metabolism, and receptor sensitivity can affect the required dose. The calculated dose should always be verified with clinical evaluation and laboratory follow-up.

Why is T3 often prescribed to be taken multiple times a day?
T3 has a very short half-life of about one day. Taking the total daily dose at once creates a sharp spike in blood levels followed by a rapid drop. Splitting the dose helps maintain more stable hormone levels throughout the day.

What are the main risks when switching from T4 to T3?
The primary risk is inducing symptoms of hyperthyroidism (thyrotoxicosis), such as heart palpitations, anxiety, or insomnia, due to T3’s high potency and rapid action. Close monitoring is essential to mitigate this risk.

Is switching from T4 to T3 a common treatment for hypothyroidism?
No, T4 monotherapy (levothyroxine) remains the standard of care. T3 monotherapy is typically reserved for short-term use in specific situations, such as preparing a thyroid cancer patient for radioactive iodine therapy or treating myxedema coma.

What lab tests are needed after changing my thyroid medication?
Your healthcare provider will typically order a TSH (Thyroid-Stimulating Hormone) test 4-6 weeks after the switch. They may also check Free T4 and Free T3 levels to ensure you are within the therapeutic range.

Does this conversion calculator apply to Natural Desiccated Thyroid (NDT) products?
No. NDT products (like Armour Thyroid) contain both T4 and T3 in a fixed ratio. This calculator is only for converting between synthetic T4 (levothyroxine) and synthetic T3 (liothyronine) monotherapies.

1. Garber, J. R., Cobin, R. H., Gharib, H., et al. (2012). Clinical practice guidelines for hypothyroidism in adults: cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association. Endocrine Practice, 18(6), 988-1028. DOI: 10.4158/EP12280.GL
2. 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. DOI: 10.1089/thy.2014.0028
3. U.S. Food and Drug Administration. SYNTHROID (levothyroxine sodium) tablets, for oral use. Prescribing Information. Access FDA Label
4. U.S. Food and Drug Administration. CYTOMEL (liothyronine sodium) tablets, for oral use. Prescribing Information. Access FDA Label