EUTHROID-3
Clinical safety rating: caution
Comprehensive clinical and safety monograph for EUTHROID-3 (EUTHROID-3).
EUTHROID-3 is a combination of liothyronine (T3) and levothyroxine (T4) that supplements endogenous thyroid hormone. T4 is converted to the active T3 in peripheral tissues. T3 binds to thyroid hormone receptors in the cell nucleus, modulating gene transcription and increasing metabolism, protein synthesis, and oxygen consumption.
| Metabolism | Levothyroxine (T4) is metabolized to liothyronine (T3) via deiodination in peripheral tissues (liver, kidney, etc.). Liothyronine (T3) is metabolized via deiodination and conjugation (glucuronidation and sulfation) in the liver and kidneys. Hepatic enzymes involved include deiodinases (D1, D2) and UDP-glucuronosyltransferases (UGTs). |
| Excretion | Renal (approx. 20-40% as unchanged drug and metabolites), biliary/fecal (approx. 60-80% as conjugated metabolites). |
| Half-life | L-T4: 6-7 days; L-T3: 1-2 days. Clinical context: Steady-state achieved in ~6 weeks for T4, ~8 days for T3. |
| Protein binding | 99.8% for L-T4 (thyroxine-binding globulin, transthyretin, albumin); 99.7% for L-T3 (same proteins, lower affinity). |
| Volume of Distribution | L-T4: 0.1-0.2 L/kg (mainly intravascular); L-T3: 0.4-0.6 L/kg (broader tissue distribution). |
| Bioavailability | Oral L-T4: 80-90% (fasting; reduced by food and malabsorption). Oral L-T3: 95-100% (well absorbed). |
| Onset of Action | Oral L-T4: 3-5 days for detectable serum T4 increase, 6-12 weeks for full therapeutic effect. Oral L-T3: 6-12 hours for peak effect. |
| Duration of Action | L-T4: 2-4 weeks after discontinuation due to long half-life. L-T3: 24-72 hours; requires multiple daily dosing for stable effect. |
| Molecular Weight | 776.87 |
Levothyroxine/liothyronine combination (EUTHROID-3): 1 tablet (50 mcg levothyroxine, 15 mcg liothyronine) orally once daily, adjusted based on TSH levels.
| Dosage form | TABLET |
| Renal impairment | No specific GFR-based dose adjustment required; monitor thyroid function in severe chronic kidney disease (GFR <30 mL/min/1.73 m²) as drug clearance may be reduced. |
| Liver impairment | No specific adjustment for Child-Pugh class A or B; use with caution in Child-Pugh C due to reduced hepatic conversion, monitor TSH. |
| Pediatric use | Not FDA-approved for children; adult dose not suitable. For hypothyroidism in children, use levothyroxine monotherapy at 25-50 mcg/day for ages 1-3 years, 50-100 mcg/day for ages 3-10 years, and 100-150 mcg/day for ages 10-16 years, adjusted per TSH. |
| Geriatric use | Start with lower dose: 25 mcg levothyroxine/7.5 mcg liothyronine (half tablet) orally once daily, titrate slowly every 4-6 weeks based on TSH, due to increased risk of cardiac adverse effects and altered metabolism. |
| 1st trimester | Thyroid hormones are essential for fetal brain development in first trimester; however, maternal hypothyroidism should be treated with appropriate doses. Exogenous liothyronine and levothyroxine do not cross placenta significantly, but excessive doses may suppress fetal TSH. Use only if clearly needed. |
| 2nd trimester | Similar to first trimester. Maternal requirements increase during pregnancy; monitor thyroid function tests and adjust dose. No known teratogenicity at therapeutic doses. |
| 3rd trimester | Continue as needed to maintain euthyroid state. Minimal placental transfer of exogenous thyroid hormones; no known adverse fetal effects at therapeutic doses. |
Clinical note
Comprehensive clinical and safety monograph for EUTHROID-3 (EUTHROID-3).
| Placental transfer | Minimal placental transfer; endogenous and exogenous thyroid hormones cross poorly due to placental deiodinase activity and binding proteins. |
| Breastfeeding |
■ FDA Black Box Warning
None
| Serious Effects |
Untreated thyrotoxicosisUncorrected adrenal insufficiencyRecent myocardial infarctionHypersensitivity to any component
| Precautions | Cardiac toxicity (e.g., arrhythmias, angina, myocardial infarction) due to excessive thyroid hormone levels, Thyrotoxic crisis (thyroid storm) if overdosed, Adrenal insufficiency: may precipitate acute adrenal crisis in patients with adrenal insufficiency, Delayed bone maturation in children if overtreated, Interactions with anticoagulants (increased INR), oral antidiabetic agents (hyperglycemia), and catecholamines (sympathomimetic effects) |
| Food/Dietary | Take on an empty stomach with water. Avoid concurrent intake with high-fiber foods, walnuts, soybean flour, cottonseed meal, or calcium/iron supplements within 4 hours of dosing as they may reduce absorption. |
Loading safety data…
| Exogenous thyroid hormones (levothyroxine and liothyronine) are excreted into breast milk in low amounts, which are not sufficient to cause adverse effects in nursing infants or alter neonatal thyroid function. Monitor maternal thyroid status and infant for excessive thyroid hormone effects if high doses are used. |
| Lactation Rating | L1 |
| Teratogenic Risk | Liothyronine (T3) and levothyroxine (T4) are endogenous thyroid hormones. Inadequate maternal thyroid hormone levels are teratogenic. At therapeutic doses, no known teratogenic risk from exogenous thyroid hormone. Fetal thyroid function develops at 10-12 weeks; prior to that, fetus depends on maternal T4. Overdose may cause fetal thyrotoxicosis. First trimester: maternal hypothyroidism increases risk of miscarriage and neurodevelopmental deficits. Second/third trimester: overtreatment may cause fetal tachycardia and growth restriction. Postpartum: adjust dose to prevent maternal hypothyroidism. |
| Fetal Monitoring | Monitor maternal TSH and free T4 every 4-6 weeks during pregnancy, more frequently if dose adjusted. Assess fetal growth and heart rate if maternal thyroid status unstable. Neonatal thyroid screening recommended. Postpartum: reassess thyroid function within 4-6 weeks. |
| Fertility Effects | Correction of hypothyroidism restores ovulation and improves fertility. No direct adverse effects on fertility from therapeutic doses. Untreated hypothyroidism may cause anovulatory cycles and infertility. |
| Clinical Pearls | Euthroid-3 is a combination of liothyronine (T3) and levothyroxine (T4) in a fixed 1:4 ratio. Monitor TSH, free T4, and free T3 levels to avoid iatrogenic hyperthyroidism. Adjust dose cautiously in elderly or cardiac patients. Use with caution in adrenal insufficiency as thyroid replacement can precipitate adrenal crisis. |
| Patient Advice | Take exactly as prescribed, typically once daily on an empty stomach 30-60 minutes before breakfast. · Do not switch between different thyroid hormone products without consulting your doctor. · Report symptoms of hyperthyroidism (rapid heartbeat, chest pain, heat intolerance, excessive sweating) or hypothyroidism (fatigue, weight gain, cold intolerance). · Inform all healthcare providers you are taking this medication. · Store at room temperature away from light and moisture. |