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. |
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 | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for EUTHROID-3 (EUTHROID-3).
| Breastfeeding | Excreted in human milk in low amounts. T3 and T4 are endogenous hormones; exogenous administration results in minimal transfer. M/P ratio: not established for Euthroid-3, but for levothyroxine, M/P ratio ~0.001. Considered compatible with breastfeeding when used at recommended doses. Monitor infant for thyroid suppression (rare at maternal therapeutic doses). |
| 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. |
■ FDA Black Box Warning
None
| Serious Effects |
["Untreated adrenal insufficiency","Thyrotoxicosis (any etiology)","Acute myocardial infarction (recent)","Hypersensitivity 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)"] |
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| 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. |