THYROLAR-1
Clinical safety rating: caution
Comprehensive clinical and safety monograph for THYROLAR-1 (THYROLAR-1).
Thyrolar-1 is a combination of levothyroxine (T4) and liothyronine (T3). T4 is converted to the active hormone T3 in peripheral tissues. Both forms bind to thyroid hormone receptors, which regulate gene transcription, influencing metabolism, growth, and development.
| Metabolism | Levothyroxine is metabolized primarily by deiodination in peripheral tissues via iodothyronine deiodinases (D1, D2, D3). Liothyronine is mainly deiodinated by D3. Hepatic metabolism includes conjugation (glucuronidation, sulfation) and biliary excretion. |
| Excretion | Renal excretion of iodide; after deiodination of T3 and T4, iodine is excreted in urine (∼80%) and feces (∼20%). |
| Half-life | Levothyroxine (T4): 6–7 days; Liothyronine (T3): 1–2 days. In hyperthyroidism, T4 half-life may be reduced to 3–4 days; in hypothyroidism, prolonged to 9–10 days. |
| Protein binding | >99% bound primarily to thyroxine-binding globulin (TBG), also transthyretin and albumin. |
| Volume of Distribution | Levothyroxine: 0.15–0.2 L/kg; Liothyronine: 0.4 L/kg. Reflects extensive tissue distribution and binding. |
| Bioavailability | Oral: approximately 80–95% for levothyroxine; liothyronine is nearly 100% absorbed. |
| Onset of Action | Oral: detectable effects within 3–5 days; full clinical effect requires 2–4 weeks repeated dosing. |
| Duration of Action | Levothyroxine: 3–4 weeks after discontinuation; Liothyronine: 2–3 days after discontinuation. |
| Molecular Weight | 776.87 |
Oral: 30-60 mg liothyronine (T3) daily, typically initiated at 15 mg/day and titrated upward based on clinical response. Usual maintenance dose 25-50 mg/day.
| Dosage form | TABLET |
| Renal impairment | No specific dose adjustment required for renal impairment; monitor thyroid function tests as use is not recommended in severe renal failure due to decreased clearance of thyroid hormones. |
| Liver impairment | Use with caution in hepatic impairment; reduce starting dose to 15 mg/day and titrate slowly; monitor thyroid function and adjust based on response. |
| Pediatric use | Not recommended for children due to lack of safety and efficacy data; alternative therapies preferred. |
| Geriatric use | Initiate at 15 mg/day; increase by 5-10 mg/day every 2-4 weeks; monitor cardiovascular status closely due to increased sensitivity to thyroid hormones. |
| 1st trimester | Use only if clearly needed; adjust dose to maintain euthyroidism. Crosses placenta minimally. |
| 2nd trimester | Continue therapy; dose may need adjustment. Monitor thyroid function. |
| 3rd trimester | Continue therapy; dose may need adjustment. Monitor thyroid function. |
Clinical note
Comprehensive clinical and safety monograph for THYROLAR-1 (THYROLAR-1).
| Placental transfer | Minimal; levothyroxine crosses placenta poorly due to deiodinase activity. |
| Breastfeeding | Excreted into breast milk in low amounts; not expected to cause adverse effects in infants. Compatible with breastfeeding. |
| Lactation Rating |
■ FDA Black Box Warning
Not for use in obesity or weight loss; ineffective and dangerous at high doses, especially with cardiovascular disease.
| Serious Effects |
Untreated thyrotoxicosisAcute myocardial infarctionUncorrected adrenal insufficiency
| Precautions | Cardiovascular risks with over-treatment (tachycardia, arrhythmia, angina, myocardial infarction), Adrenal insufficiency crisis if used in untreated adrenal insufficiency, Thyrotoxicosis factitia from excessive doses, Increased bone resorption and osteoporosis risk with long-term TSH suppression, Interactions with anticoagulants, diabetes medications, beta-blockers |
| Food/Dietary | Avoid high-fiber foods, walnuts, soybean flour, cottonseed meal, and high-calcium foods (e.g., dairy) within 4 hours of dosing as they may reduce absorption. Iron supplements, calcium carbonate, aluminum antacids, and sevelamer should be separated by at least 4 hours. Grapefruit juice may inhibit T4 metabolism; limit intake. |
Loading safety data…
| L1: Safest |
| Teratogenic Risk | First trimester: No evidence of teratogenicity; maternal hypothyroidism itself may increase risk of fetal loss and neurodevelopmental deficits. Second/third trimester: No direct teratogenic effect; adequate maternal thyroid hormone levels are critical for fetal neurodevelopment. |
| Fetal Monitoring | Monitor maternal TSH and free T4 every 4-6 weeks during pregnancy; adjust dose to maintain TSH in trimester-specific reference range. Fetal monitoring: assess heart rate, growth; consider fetal thyroid function in cases of maternal overtreatment. |
| Fertility Effects | Untreated maternal hypothyroidism can cause ovulatory dysfunction and infertility; euthyroid state improves fertility outcomes. No direct adverse effects on fertility with proper dosing. |
| Clinical Pearls | THYROLAR-1 is a desiccated thyroid extract (DTE) containing both T4 and T3. Monitor TSH, free T4, and free T3 levels; T3 may cause supraphysiologic peaks. Start with low doses (e.g., 15 mg) and titrate every 2-4 weeks. Contraindicated in untreated adrenal insufficiency. Use with caution in cardiac disease; may increase heart rate and exacerbate angina. Not FDA-approved; use only if patient fails synthetic T4 (levothyroxine). |
| Patient Advice | Take exactly as prescribed, usually once daily on an empty stomach 30-60 minutes before breakfast with a full glass of water. · Do not switch brands or between desiccated thyroid and levothyroxine without consulting your doctor. · Report symptoms of hyperthyroidism (rapid heartbeat, anxiety, weight loss, sweating) or hypothyroidism (fatigue, weight gain, cold intolerance). · Keep all appointments for blood tests (TSH, T4) to adjust dose. · Store at room temperature away from moisture and heat. · If you miss a dose, take it as soon as remembered unless close to next dose; do not double. |