THYROLAR-5
Clinical safety rating: caution
Comprehensive clinical and safety monograph for THYROLAR-5 (THYROLAR-5).
Thyrolar-5 is a combination of levothyroxine (T4) and liothyronine (T3), synthetic thyroid hormones. T4 is converted to T3 in peripheral tissues. T3 binds to thyroid hormone receptors, regulating gene transcription and increasing cellular metabolism.
| Metabolism | Levothyroxine (T4) is deiodinated to T3 primarily in liver, kidney, and muscle via iodothyronine deiodinases. Liothyronine (T3) undergoes hepatic conjugation and excretion. Both are partially metabolized via glucuronidation and sulfation. |
| Excretion | Renal: 40-50% (as conjugated metabolites); Fecal: 20-30% (enterohepatic recirculation); Biliary: minor |
| Half-life | Liothyronine (T3): 1-2 days; Levothyroxine (T4): 6-7 days. Clinical context: In hyperthyroidism, T4 half-life shortens to 3-4 days; in hypothyroidism, prolongs to 9-10 days |
| Protein binding | >99% bound; Thyroxine-binding globulin (TBG) 70%, transthyretin (TTR) 10-15%, albumin 15-20% |
| Volume of Distribution | Levothyroxine: 0.1-0.2 L/kg (extracellular and intracellular compartments); Liothyronine: 0.4-0.5 L/kg (larger due to active cellular uptake). Clinical meaning: Higher Vd for T3 reflects rapid equilibration with tissues |
| Bioavailability | Oral: Levothyroxine 40-80% (variable, affected by food and GI conditions); Liothyronine 95% (well absorbed) |
| Onset of Action | Oral: Therapeutic effect (improvement in hypothyroid symptoms) within 3-5 days for T3, 1-2 weeks for T4. IV: Not applicable (oral only) |
| Duration of Action | Levothyroxine: 1-3 weeks after single T4 dose; Liothyronine: 24-72 hours. Clinical note: T4 provides stable hormone levels; T3 has shorter duration and requires divided doses |
| Molecular Weight | 776.87 (levothyroxine), 650.98 (liothyronine) per respective components. Combined product: average molecular weight ~713.9 Da. |
Oral, starting dose 15-30 mg daily, titrated to maintenance dose of 60-120 mg daily, divided into 2-3 doses.
| Dosage form | TABLET |
| Renal impairment | GFR 30-60 mL/min: reduce dose by 25-50%. GFR 15-29 mL/min: reduce dose by 50-75%. GFR <15 mL/min: use with caution, reduce dose by 75% or consider alternative. |
| Liver impairment | Child-Pugh A: no adjustment. Child-Pugh B: reduce dose by 50%. Child-Pugh C: avoid use or reduce dose by 75% with close monitoring. |
| Pediatric use | Not recommended for children <12 years due to lack of safety data. For children ≥12 years: initial dose 0.5 mg/kg/day oral, titrate to 1-3 mg/kg/day, max 120 mg/day. |
| Geriatric use | Start at lowest dose (15 mg daily), titrate slowly with monitoring for hypotension and CNS effects. Consider reduced maintenance doses due to age-related decrease in clearance. |
| 1st trimester | Thyroid hormones are essential for fetal development. Levothyroxine and liothyronine are not associated with teratogenic risk at therapeutic doses. Monitor thyroid function closely and adjust dose to maintain euthyroid state. |
| 2nd trimester | Continue therapy under close monitoring. Increased maternal thyroid hormone requirements may necessitate dose adjustments. Untreated hypothyroidism poses risks to both mother and fetus. |
| 3rd trimester | Continue therapy. Thyroid hormones cross the placenta minimally and are crucial for fetal neurodevelopment. Adjust dose as needed based on thyroid function tests. |
Clinical note
Comprehensive clinical and safety monograph for THYROLAR-5 (THYROLAR-5).
| Placental transfer | Minimal; levothyroxine and liothyronine cross the placenta in limited amounts. Endogenous thyroid hormones are regulated by placental deiodinases and transporters. |
| Breastfeeding | Small amounts of thyroid hormones are excreted into breast milk but are not expected to cause adverse effects in the infant at therapeutic maternal doses. Monitor infant for signs of hyperthyroidism if mother is on high doses. Thyroid supplementation is considered compatible with breastfeeding. |
■ FDA Black Box Warning
Not effective for weight reduction; serious cardiovascular toxicity may occur when used in therapeutic doses for obesity, especially in patients with cardiovascular disease or when taken with other anorectic agents.
| Serious Effects |
Untreated thyrotoxicosisUntreated adrenal insufficiencyHypersensitivity to any component
| Precautions | Cardiac adverse reactions (angina, arrhythmias, heart failure); may increase risk of atrial fibrillation; use with caution in elderly, patients with cardiovascular disease, or adrenal insufficiency; monitor thyroid function tests periodically; dose adjustments may be needed during pregnancy or concomitant medications (e.g., oral contraceptives, antidiabetic agents). |
| Food/Dietary | Take on empty stomach with water; avoid food, coffee, or milk within 30-60 minutes. High-fiber foods, soy, walnuts, cottonseed meal, and calcium/iron supplements may decrease absorption. Grapefruit juice may increase liothyronine absorption. Maintain consistent dietary habits. |
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| Lactation Rating | L2 (Safer) |
| Teratogenic Risk | Thyroid hormones have low teratogenic potential. In first trimester, maternal hypothyroidism is associated with increased risk of miscarriage and fetal neurodevelopmental deficits. In second and third trimesters, untreated hypothyroidism risks fetal growth restriction, preterm birth, and impaired cognitive development. Exogenous thyroid hormone replacement is considered safe and essential. No evidence of teratogenicity from therapeutic doses. |
| Fetal Monitoring | Monitor maternal thyroid function (TSH, free T4) every 4-6 weeks during pregnancy, and every 2-4 weeks after dose adjustments. Assess fetal growth and heart rate. Consider neonatal TSH screening at birth. |
| Fertility Effects | Hypothyroidism can impair fertility (anovulation, menstrual irregularities). Euthyroid state with thyroid hormone replacement restores fertility. No direct adverse effect of THYROLAR-5 on fertility. |
| Clinical Pearls | Thyrolar-5 is a combination levothyroxine (T4) and liothyronine (T3) product with a T4:T3 ratio of approximately 4:1. Initiate at 12.5-25 mcg daily; monitor TSH, FT4, FT3 after 4-6 weeks. Use cautiously in elderly, patients with cardiac disease, or adrenal insufficiency. T3 component may cause more rapid onset of action and potential for thyrotoxic symptoms. Avoid in pregnancy (T3 crosses placenta poorly; use T4 monotherapy). |
| Patient Advice | Take exactly as prescribed, usually once daily on an empty stomach, 30-60 minutes before breakfast. · Do not switch between different thyroid hormone products unless directed by your doctor, as potency differs. · Report symptoms of hyperthyroidism (rapid heartbeat, chest pain, nervousness, excessive sweating) or hypothyroidism (fatigue, weight gain, cold intolerance, constipation). · Consistency in timing and dosage is critical; skip missed dose if remembered later in the day, but do not double. · Inform all healthcare providers you are taking this medication; it may interact with warfarin, beta-blockers, and certain seizure medications. |