THYROLAR-2
Clinical safety rating: caution
Comprehensive clinical and safety monograph for THYROLAR-2 (THYROLAR-2).
Thyrolar-2 is a combination of levothyroxine (T4) and liothyronine (T3), synthetic thyroid hormones. T4 is converted to the active T3 in peripheral tissues. T3 binds to thyroid hormone receptors in the nucleus, modulating gene transcription and increasing metabolic rate, oxygen consumption, and protein synthesis.
| Metabolism | Levothyroxine (T4) is primarily deiodinated to triiodothyronine (T3) in peripheral tissues (liver, kidney, etc.) by deiodinase enzymes (D1, D2). T3 is further metabolized by deiodination and conjugation (glucuronidation, sulfation). T4 and T3 undergo enterohepatic recirculation. CYP450 enzymes play a minor role. |
| Excretion | Renal: 40% (as glucuronide and sulfate conjugates); Fecal: 20% (unabsorbed); Biliary: minor (<5%) |
| Half-life | 6-7 days (euthyroid); clinical context: steady-state reached in 4-6 weeks |
| Protein binding | 99.97% bound to thyroxine-binding globulin (TBG), transthyretin, and albumin |
| Volume of Distribution | 0.2-0.3 L/kg; reflects distribution into total body water |
| Bioavailability | Oral: 80-90% (fasting); reduced by food, fiber, and iron supplements |
| Onset of Action | Oral: 3-5 days; IV: 12-24 hours |
| Duration of Action | 5-7 days after discontinuation; prolonged in hypothyroidism |
| Molecular Weight | 776.93 |
1/2 to 1 tablet (30-60 mg liotrix) orally once daily, titrated every 2-4 weeks by 1/2 tablet increments based on clinical response and thyroid function tests.
| Dosage form | TABLET |
| Renal impairment | No specific GFR-based dose modification required; however, monitor thyroid function closely in severe renal impairment (GFR <30 mL/min) due to altered binding and metabolism. |
| Liver impairment | For Child-Pugh Class A or B: reduce starting dose by 50% and titrate slowly. For Child-Pugh Class C: avoid use or initiate at 25% of usual dose with close monitoring. |
| Pediatric use | Initial dose: 12.5-25 mcg of levothyroxine component (or 15-30 mg liotrix) once daily, adjusted based on weight (2-3 mcg/kg/day levothyroxine equivalents). For children >12 years, use adult dosing with caution. |
| Geriatric use | Initiate at 12.5-25 mcg levothyroxine equivalents (1/4 to 1/2 tablet) once daily; increase by 12.5 mcg every 4-6 weeks. Monitor for cardiovascular side effects and avoid over-replacement. |
| 1st trimester | Avoid if possible; may cause fetal thyrotoxicosis. Use only if clearly needed. |
| 2nd trimester | Use with caution; monitor fetal heart rate and growth. |
| 3rd trimester | Use with caution; may cause fetal goiter or hyperthyroidism. |
Clinical note
Comprehensive clinical and safety monograph for THYROLAR-2 (THYROLAR-2).
| Placental transfer | Limited placental transfer; less than 10% of maternal dose. |
| Breastfeeding | Excreted into breast milk in low amounts; monitor infant for signs of hyperthyroidism. |
| Lactation Rating | L3 - Moderate Safety |
■ FDA Black Box Warning
None
| Serious Effects |
Untreated hyperthyroidismUntreated adrenal insufficiencyHypersensitivity to any component
| Precautions | Not for use in the treatment of obesity or weight loss; caution in patients with cardiovascular disease (e.g., angina, arrhythmias, hypertension) due to increased myocardial oxygen demand; thyroid hormone can exacerbate hyperthyroidism or thyroid storm; monitor for signs of thyrotoxicosis; adjust dose in elderly and patients with adrenal insufficiency; may cause decreased bone mineral density with long-term use; interactions with anticoagulants, antidiabetic agents, and other drugs that affect thyroid function. |
| Food/Dietary | Avoid taking with high-fiber foods, calcium-fortified juices, or soy products within 4 hours of dose. Grapefruit juice may increase absorption; avoid concurrent use. Iron and calcium supplements should be separated by at least 4 hours. Caffeine may decrease absorption; avoid simultaneous intake. |
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| Teratogenic Risk | THYROLAR-2 (liothyronine/levothyroxine combination) is thyroid hormone replacement. Fetal risks: First trimester: No known teratogenic effects; physiological replacement is essential for fetal neurodevelopment. Second and third trimesters: No known fetal risks; necessary for maternal and fetal metabolic homeostasis. Hypothyroidism (untreated) poses significant risks: miscarriage, preeclampsia, fetal neurodevelopmental deficits. |
| Fetal Monitoring | Monitor maternal thyroid function (TSH, free T4, free T3) every 4-6 weeks during pregnancy and postpartum. Adjust dose to maintain TSH within trimester-specific reference ranges. Fetal monitoring: fetal heart rate, growth ultrasound if maternal thyroid dysfunction is present. Consider neonatal thyroid screening at birth if maternal thyroid disease. |
| Fertility Effects | Untreated or undertreated hypothyroidism is associated with ovulatory dysfunction, infertility, and increased miscarriage risk. Thyroid hormone replacement with THYROLAR-2 restores euthyroid state, improving fertility outcomes. No direct adverse fertility effects of the drug at therapeutic doses. |
| Clinical Pearls | Thyrolar-2 combines levothyroxine (T4) and liothyronine (T3) in a fixed 4:1 ratio. Avoid use in patients with untreated thyrotoxicosis. Monitor thyroid function tests every 6-8 weeks after dose changes. T3 component may cause rapid symptom relief but also increased risk of palpitations. Dose adjustments should be small (12.5-25 mcg increments) due to narrow therapeutic index. |
| Patient Advice | Take exactly as prescribed, usually once daily on an empty stomach 30-60 minutes before breakfast. · Do not stop or change dose without consulting your doctor. · Report symptoms of hyperthyroidism (rapid heartbeat, chest pain, nervousness, excessive sweating) or hypothyroidism (fatigue, weight gain, cold intolerance). · Consistency in brand and dosing is important; avoid switching between different thyroid products without medical supervision. · Store at room temperature, away from light and moisture. |