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. |
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 | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for EUTHROID-2 (EUTHROID-2).
| Breastfeeding | Minimal excretion into breast milk. Both levothyroxine and liothyronine are endogenous hormones; exogenous doses result in negligible transfer. Milk-to-plasma ratio (M/P) < 0.01 for levothyroxine; liothyronine M/P ~0.3. Not expected to cause adverse effects in breastfed infants at usual maternal doses. No contraindication to breastfeeding with appropriate thyroid monitoring. |
| 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. |
■ 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 |
["Hypersensitivity to any component of the product","Untreated or inadequately treated adrenal insufficiency","Untreated thyrotoxicosis (hyperthyroidism)","Recent myocardial infarction (relative contraindication due to risk of cardiac ischemia)","Concurrent use of sympathomimetic amines (e.g., for weight loss) may increase cardiac risk"]
| 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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| 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. |