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 |
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 | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for THYROLAR-5 (THYROLAR-5).
| Breastfeeding | Small amounts of thyroid hormones are excreted into breast milk. M/P ratio not well-defined. Levels are insufficient to affect infant thyroid function or cause adverse effects. Replacement therapy is considered compatible with breastfeeding when used at maternal requirement doses. |
| 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. |
■ 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 thyrotoxicosis; uncorrected adrenal insufficiency; recent myocardial infarction; hypersensitivity 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). |
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| 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. |