EUTHROID-2
Clinical safety rating: caution
Comprehensive clinical and safety monograph for EUTHROID-2 (EUTHROID-2).
EUTHROID-2 is a synthetic formulation of liothyronine (T3) and levothyroxine (T4) that replaces 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 to increase metabolic rate, oxygen consumption, and protein, carbohydrate, and lipid metabolism.
| Metabolism | Levothyroxine (T4) is metabolized via deiodination by type 1 and type 2 deiodinases in peripheral tissues to the active form liothyronine (T3) and to reverse T3 (rT3). Further metabolism involves conjugation (glucuronidation and sulfation) in the liver and excretion in bile and urine. |
| Excretion | Renal: ~20-40% of T4 and T3 metabolites; fecal: ~40-60% as conjugated metabolites; minor biliary elimination. |
| Half-life | T4: 6-7 days (euthyroid); T3: approximately 1 day; clinical context: requires 6-8 weeks for steady state with T4 therapy. |
| Protein binding | T4: >99.95% bound to TBG, TTR, albumin; T3: ~99.7% bound to same proteins; free fraction T4 ~0.03%, T3 ~0.3%. |
| Volume of Distribution | T4: 0.1-0.2 L/kg (small); T3: 0.4-0.6 L/kg (larger due to less protein binding); clinical: reflects extensive tissue distribution for T3. |
| Bioavailability | Oral: T4 70-80% (fasting, consistent); T3 90-95%; IV: 100%. |
| Onset of Action | Oral: T4 onset 3-5 days; T3 onset 2-4 hours; IV: T4 onset 6-8 hours; T3 onset within 1 hour. |
| Duration of Action | T4: 2-3 weeks; T3: 1-2 days; clinical note: T4 provides stable levels, T3 used for rapid correction. |
| Molecular Weight | 776.87 (levothyroxine), 672.96 (liothyronine); combined product composition varies, but active components' MWs are as stated. |
Oral, 1 tablet once daily. Each tablet contains levothyroxine 112 mcg and liothyronine 28.8 mcg.
| Dosage form | TABLET |
| Renal impairment | No dose adjustment required for mild to moderate renal impairment. For severe renal impairment (GFR < 15 mL/min), monitor thyroid function closely and consider dose reduction by 25%. |
| Liver impairment | Child-Pugh A: No adjustment. Child-Pugh B: Reduce dose by 25%. Child-Pugh C: Reduce dose by 50% or avoid use. |
| Pediatric use | Weight-based dosing (levothyroxine equivalent): 1-2 mcg/kg/day orally. For neonates (0-3 months): 10-15 mcg/kg/day. Adjust based on TSH and free T4 levels. |
| Geriatric use | Start with lower dose (levothyroxine equivalent 25-50 mcg/day) and titrate slowly. Monitor for cardiac effects due to increased sensitivity. |
| 1st trimester | Levothyroxine and liothyronine are FDA pregnancy category A. Thyroid hormones do not readily cross placenta; minimal fetal exposure. Recommended for maternal hypothyroidism; euthyroid state critical for fetal neurodevelopment. |
| 2nd trimester | Same as first trimester; continued monitoring and dose adjustments as needed to maintain euthyroid state. |
| 3rd trimester | Same as first and second trimesters; increased requirements may occur; monitor thyroid function periodically. |
Clinical note
Comprehensive clinical and safety monograph for EUTHROID-2 (EUTHROID-2).
| Placental transfer | Minimal; levothyroxine and liothyronine cross the placenta poorly. Fetal exposure is low and not clinically significant at therapeutic doses. |
| Breastfeeding | Levothyroxine and liothyronine are distributed into breast milk in low amounts. Exogenous thyroid hormones do not appear to cause adverse effects in nursing infants. Monitor infant thyroid function if mother is on high doses. |
■ FDA Black Box Warning
No FDA boxed warning. However, inappropriate use (e.g., for obesity or weight loss) in euthyroid patients is dangerous and can cause serious or life-threatening toxicity, especially when combined with sympathomimetic amines.
| Serious Effects |
Untreated adrenal insufficiency (thyroid hormones increase metabolic demand and may precipitate acute adrenal crisis)Untreated thyrotoxicosis (relative contraindication; use with extreme caution)Hypersensitivity to any component
| Precautions | Cardiac toxicity: Risk of tachyarrhythmias, angina, myocardial ischemia in patients with cardiovascular disease; start with low doses and titrate slowly, Thyrotoxic crisis: Accidental overdose may cause thyrotoxicosis or thyroid storm; monitor for symptoms of hyperthyroidism (tachycardia, chest pain, nervousness, insomnia), Adrenal insufficiency: Thyroid hormone therapy may increase cortisol clearance and precipitate acute adrenal crisis in patients with adrenal insufficiency; treat adrenal insufficiency prior to thyroid replacement, Osteoporosis: Long-term excessive thyroid hormone may cause decreased bone mineral density, Diabetes: May alter glucose metabolism; monitor blood glucose in diabetic patients, Warfarin interaction: Thyroid hormone potentiates anticoagulant effect of warfarin; reduce warfarin dose upon initiation of thyroid therapy |
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| Lactation Rating | L1 (Most Compatible) |
| Teratogenic Risk | EUTHROID-2 (levothyroxine 100 mcg + liothyronine 20 mcg) is a combination thyroid hormone replacement. Hypothyroidism itself increases risk of miscarriage and fetal neurodevelopmental deficits if untreated. Levothyroxine and liothyronine do not cross the placenta in significant amounts at physiological doses and are not associated with congenital malformations. No teratogenic effects in first trimester. In second and third trimesters, maternal euthyroidism is critical; undertreatment may lead to fetal goiter, impaired neurological development, or preterm birth. Overtreatment carries risk of maternal tachycardia, arrhythmia, and potential fetal thyrotoxicosis. The benefit of treating maternal hypothyroidism outweighs risks. |
| Fetal Monitoring | Monitor maternal thyroid function (TSH, free T4) every 4-6 weeks during pregnancy and postpartum; adjust dose to maintain TSH within trimester-specific reference range (first trimester 0.2-2.5 mIU/L, second trimester 0.3-3.0 mIU/L, third trimester 0.5-3.5 mIU/L). Fetal monitoring: ultrasound for growth, heart rate, and goiter if maternal thyroid dysfunction suspected. After birth, screen neonatal thyroid function if maternal antibodies present or if high doses used. |
| Fertility Effects | Hypothyroidism is associated with ovulatory dysfunction, menstrual irregularities, and decreased fertility. Euthyroid state restoration improves conception rates and reduces early pregnancy loss. No direct negative effect on fertility from the drug itself. |
| Food/Dietary | Avoid high-fiber foods, soy products, walnuts, grapefruit juice, and high-calcium foods (milk, yogurt) at the time of dosing as they can impair absorption. Take medication at least 30 minutes before meals. Foods containing goitrogens (e.g., cruciferous vegetables like broccoli, cabbage, kale) in large amounts may interfere with thyroid function but are generally not a concern with adequate iodine intake. |
| Clinical Pearls | Euthroid-2 is a synthetic combination of levothyroxine (T4) and liothyronine (T3) used for thyroid hormone replacement. Monitor TSH levels 6-8 weeks after dose changes; target TSH within normal range. T3 component may cause more rapid symptom relief but also risk of iatrogenic thyrotoxicosis if overdosed. Use with caution in elderly, cardiac disease, or adrenal insufficiency. Avoid abrupt discontinuation. Starting dose typically 50-100 mcg T4 equivalent; adjust per TSH. T3 half-life ~1 day vs T4 ~7 days; twice-daily dosing may be considered for T3 but Euthroid-2 is usually dosed once daily. Drug interactions: warfarin (increased INR), antidiabetic agents (need dose adjustment), beta-blockers (reduce T4 to T3 conversion). |
| Patient Advice | Take Euthroid-2 on an empty stomach, at least 30 minutes before breakfast or 2 hours after a meal, with a full glass of water. · Do not discontinue medication abruptly; consult your doctor before stopping. · Report symptoms of hyperthyroidism (rapid heartbeat, anxiety, tremors, weight loss, heat intolerance) or hypothyroidism (fatigue, weight gain, cold intolerance, depression). · Avoid iron supplements, calcium supplements, antacids, and sucralfate within 4 hours of taking Euthroid-2. · Consistent timing and brand are important; do not switch to generic or different brand without doctor approval. · Pregnancy: inform your doctor if pregnant or planning; dose may need adjustment. · Regular blood tests (TSH) are required to monitor therapy. |