EUTHROID-1
Clinical safety rating: caution
Comprehensive clinical and safety monograph for EUTHROID-1 (EUTHROID-1).
Euthroid-1 is a combination of levothyroxine (T4) and liothyronine (T3), synthetic thyroid hormones that replace endogenous thyroid hormone. T4 is converted to T3 in peripheral tissues, acting on thyroid hormone receptors to regulate gene transcription, metabolism, and growth.
| Metabolism | Levothyroxine is deiodinated to liothyronine in peripheral tissues via iodothyronine deiodinases (DIO1, DIO2). Liothyronine undergoes deiodination and conjugation (glucuronidation, sulfation) in liver. |
| Excretion | Renal: ~20-40% as unchanged drug; biliary/fecal: ~40-60% as metabolites and conjugates; total clearance is primarily hepatic. |
| Half-life | Terminal elimination half-life: approximately 5-7 days for levothyroxine (T4) and 2-4 days for liothyronine (T3). Clinical context: Steady-state achieved in 6-8 weeks; half-life prolonged in hypothyroidism, shortened in hyperthyroidism. |
| Protein binding | >99% bound; T4 bound to thyroxine-binding globulin (TBG: ~70%), transthyretin (10-15%), and albumin (15-20%); T3 binds less avidly to TBG and albumin. |
| Volume of Distribution | Vd: approximately 0.1-0.2 L/kg for T4; 0.3-0.5 L/kg for T3; reflects distribution primarily into extracellular fluid and limited tissue penetration for T4, wider distribution for T3. |
| Bioavailability | Oral: 50-80% for T4 (absorption depends on formulation and food); T3 nearly completely absorbed (>90%). |
| Onset of Action | Oral: 2-4 hours for T3; T4 requires 6-12 hours; full metabolic effects in 3-5 days for T3, 10-14 days for T4. |
| Duration of Action | Clinical effects last 1-2 days for T3, 1-3 weeks for T4; requires continuous therapy for sustained effect. |
| Molecular Weight | 788.88 |
One tablet orally once daily, typically in the morning on an empty stomach. Contains 100 mcg levothyroxine and 25 mcg liothyronine.
| Dosage form | TABLET |
| Renal impairment | No specific GFR-based dose adjustment required; however, in severe renal failure, monitor thyroid function closely as drug clearance may be altered. |
| Liver impairment | No specific Child-Pugh based dose adjustment; caution in severe hepatic impairment due to altered metabolism of thyroid hormones. |
| Pediatric use | Weight-based dosing for hypothyroidism: initial 12.5-25 mcg levothyroxine equivalent per day, adjusted based on TSH and free T4 levels. Not recommended for children due to fixed combination ratio. |
| Geriatric use | Start with lower dose (e.g., half tablet) and titrate slowly; monitor for cardiac side effects due to increased sensitivity to thyroid hormones. |
| 1st trimester | Levothyroxine is considered safe; essential for maternal thyroid function and fetal neurodevelopment. Dose adjustments may be needed due to increased demand. |
| 2nd trimester | Continue therapy; monitor TSH levels and adjust dose as needed. |
| 3rd trimester | Continue therapy; monitor TSH levels and adjust dose as needed. |
Clinical note
Comprehensive clinical and safety monograph for EUTHROID-1 (EUTHROID-1).
| Placental transfer | Limited placental transfer; levothyroxine crosses placenta but endogenous levels are regulated. |
| Breastfeeding | Excreted into breast milk in low amounts; not expected to cause adverse effects in infants. Compatible with breastfeeding. |
| Lactation Rating |
■ FDA Black Box Warning
No black box warning.
| Serious Effects |
Untreated thyrotoxicosisAcute myocardial infarctionUncontrolled adrenal insufficiency
| Precautions | Cardiovascular toxicity with overdosage; may exacerbate angina, arrhythmias, hypertension. Caution in patients with diabetes mellitus (may increase blood glucose) and adrenal insufficiency. Monitor thyroid function tests and adjust dose. |
| Food/Dietary | Avoid high-fiber foods, grapefruit juice, and soy products within 4 hours of taking Euthyroid-1 as they may interfere with absorption. Maintain consistent iodine intake; avoid drastic increases in cruciferous vegetables (e.g., broccoli, kale) without medical advice. Calcium-fortified foods and iron-rich foods should be separated by at least 4 hours. |
Loading safety data…
| L1 (Safe) |
| Teratogenic Risk | EUTHROID-1 (levothyroxine) is a thyroid hormone replacement. Untreated maternal hypothyroidism is associated with increased risks of miscarriage, fetal neurodevelopmental deficits, preterm delivery, and low birth weight. Levothyroxine itself is not teratogenic; the FDA pregnancy category is A. No increased risk of congenital malformations has been reported with therapeutic doses. In the first trimester, adequate maternal T4 is critical for fetal brain development. In the second and third trimesters, placental transfer of levothyroxine is minimal as fetal thyroid function matures. Untreated hyperthyroidism from over-replacement may increase risk of fetal tachycardia, growth restriction, and preterm birth. |
| Fetal Monitoring | Maternal: Monitor serum TSH and free T4 every 4-6 weeks during pregnancy, especially in the first trimester. Adjust dose to maintain TSH within trimester-specific reference ranges. Monitor for signs of thyroid dysfunction. Fetal: Monitor fetal heart rate and growth if maternal thyroid disease is uncontrolled. In cases of overt maternal hypothyroidism or hyperthyroidism, consider fetal ultrasound for growth and thyroid gland size. Neonatal: Check newborn thyroid function as per standard screening. |
| Fertility Effects | Untreated hypothyroidism can cause anovulation, menstrual irregularities, and infertility. Restoration of euthyroidism with levothyroxine typically reverses these effects and improves fertility outcomes. There is no evidence that levothyroxine itself impairs fertility. In euthyroid women with infertility and positive thyroid antibodies, levothyroxine therapy may improve pregnancy rates, although evidence is mixed. |
| Clinical Pearls | Euthyroid-1 contains levothyroxine (T4) and liothyronine (T3) in a fixed 4:1 ratio. Monitor TSH, free T4, and free T3 levels to avoid overtreatment, especially due to T3 component. Use with caution in elderly and patients with cardiovascular disease; start with lower doses. T3 has a shorter half-life (about 1 day) vs T4 (7 days); consider this when interpreting labs. Drug interactions: iron, calcium, antacids, and bile acid sequestrants may reduce absorption; separate by at least 4 hours. |
| Patient Advice | Take exactly as prescribed at the same time each day, usually in the morning on an empty stomach with water. · Do not stop or change dose without consulting your doctor; symptoms may take weeks to improve. · Inform your doctor of all other medications and supplements you take, especially iron, calcium, and antacids. · Report symptoms of hyperthyroidism (rapid heart rate, chest pain, sweating) or hypothyroidism (fatigue, weight gain, cold intolerance). · Store at room temperature away from moisture and heat; keep out of reach of children. |