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 |
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 | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for THYROLAR-2 (THYROLAR-2).
| Breastfeeding | Thyroid hormones are endogenous and normally appear in breast milk. Liothyronine (T3) and levothyroxine (T4) enter breast milk minimally; M/P ratio not well defined for combination. Exogenous supplementation at replacement doses does not produce pharmacological levels in milk. Use during breastfeeding is considered compatible and recommended if indicated for maternal thyroid deficiency. |
| 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. |
■ FDA Black Box Warning
None
| Serious Effects |
Untreated adrenal insufficiency; untreated thyrotoxicosis; acute myocardial infarction; hypersensitivity to any ingredient; recent myocardial infarction (relative); concurrent use of certain sympathomimetics (relative).
| 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. |
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| 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. |