EUTHROID-0.5
Clinical safety rating: caution
Comprehensive clinical and safety monograph for EUTHROID-0.5 (EUTHROID-0.5).
Euthyroid-0.5 is a combination of liothyronine (T3) and levothyroxine (T4). T4 is converted to the active T3 in peripheral tissues. T3 binds to thyroid hormone receptors (TRα and TRβ) in the nucleus, regulating gene transcription involved in metabolism, growth, and development.
| Metabolism | Levothyroxine (T4) is deiodinated to liothyronine (T3) primarily by type 1 and type 2 deiodinases in liver, kidney, and other tissues. T3 and T4 are also metabolized via glucuronidation and sulfation. Hepatic enzymes: UGT1A1, UGT1A3, SULT1A1. |
| Excretion | Renal (approx. 20-40% as unchanged drug, primarily via glomerular filtration and tubular secretion); biliary/fecal (approx. 60-80% as metabolites and unchanged drug, with enterohepatic recirculation). |
| Half-life | Terminal elimination half-life is approximately 6-8 hours in adults with normal renal and hepatic function; clinically, steady-state is reached within 24-36 hours, and dosing interval adjustments may be needed in renal or hepatic impairment. |
| Protein binding | Approximately 99% bound to serum proteins, primarily thyroxine-binding globulin (TBG), with lesser binding to transthyretin and albumin. |
| Volume of Distribution | Apparent volume of distribution is approximately 0.10-0.15 L/kg, indicating distribution primarily into extracellular fluid and highly protein-bound; small Vd reflects minimal tissue binding under steady-state conditions. |
| Bioavailability | Oral bioavailability: 100% (tablets), as EUTHROID-0.5 is a combination product with synthetic T4 (levothyroxine) and T3 (liothyronine); T4 absorption is ~80% (fasting, taken with water), while T3 is nearly completely absorbed; overall bioavailability considered complete when taken as directed. |
| Onset of Action | Oral: 30-60 minutes to detectable serum levels; clinical effect (e.g., suppression of TSH or relief of symptoms) typically within 3-5 days after initiation. |
| Duration of Action | Duration of action: Up to 24 hours after a single oral dose; clinical effect persists for 1-2 weeks after discontinuation due to slow receptor dissociation and long tissue residence. Notes: Given long half-life, once-daily dosing maintains steady-state. |
Oral: 0.5 grains (30 mg) once daily, titrated to clinical response.
| Dosage form | TABLET |
| Renal impairment | No dose adjustment required for GFR >30 mL/min; for GFR <30 mL/min, consider reducing dose by 25-50% and monitor TSH. |
| Liver impairment | Child-Pugh A: no adjustment; Child-Pugh B: reduce dose by 25-50%; Child-Pugh C: avoid use or reduce dose by 50% and monitor TSH. |
| Pediatric use | Oral: 0.5-1 grain (30-60 mg) per 70 kg body weight once daily; for children <70 kg, use 0.5 grains (30 mg) once daily adjusted to TSH levels. |
| Geriatric use | Initiate at 0.5 grains (30 mg) orally once daily; titrate slowly with 0.5 grain increments every 4-6 weeks; monitor for tachyarrhythmias and osteoporosis. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for EUTHROID-0.5 (EUTHROID-0.5).
| Breastfeeding | Levothyroxine is excreted into breast milk in minimal amounts, but no adverse effects in nursing infants have been reported. The milk-to-plasma (M/P) ratio is approximately 0.5 (range 0.4-0.6). Breastfeeding is considered safe while on levothyroxine therapy. Monitor infant thyroid function if high doses are used. |
| Teratogenic Risk | EUTHROID-0.5 contains levothyroxine. Thyroid hormones are not associated with major teratogenic risk. In the first trimester, maternal hypothyroidism (treated) is important to avoid, as untreated hypothyroidism is linked to congenital anomalies and neurodevelopmental deficits. No evidence of fetal harm from levothyroxine at therapeutic doses. Second and third trimester: transfers minimal amounts across placenta, but adequate maternal levels are essential for fetal neurodevelopment. Risk of fetal goiter if mother is overtreated (TSH suppression). |
■ FDA Black Box Warning
No FDA boxed warning.
| Serious Effects |
Hypersensitivity to active ingredients or excipients. Untreated adrenal insufficiency. Thyrotoxicosis (hyperthyroidism). Acute myocardial infarction. Uncontrolled cardiovascular disease.
| Precautions | Cardiovascular effects: angina, arrhythmias, heart failure. Thyrotoxicosis: excessive doses may cause symptoms of hyperthyroidism. Bone mineral density reduction with long-term overreplacement. Adrenal insufficiency: may precipitate crisis in untreated patients. Diabetes: insulin/oral hypoglycemic requirements may increase. Myxedema coma: rapid correction can be fatal. |
| Food/Dietary | Avoid taking with high-fiber foods, soy, or calcium supplements; separate by at least 4 hours. |
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| Fetal Monitoring | Monitor maternal TSH and free T4 every 4-6 weeks during pregnancy, especially in the first trimester. Adjust dose to maintain TSH within trimester-specific reference range (0.2-2.5 mIU/L in first trimester, 0.3-3.0 in second and third). Fetal monitoring: ultrasound for growth, heart rate, and signs of goiter if maternal doses are excessive or if there is a history of thyroid dysfunction. |
| Fertility Effects | Hypothyroidism can impair fertility; with levothyroxine replacement, fertility may improve. No direct adverse effect of levothyroxine on fertility. |
| Clinical Pearls | Euthroid-0.5 contains liothyronine (T3). Monitor for signs of thyrotoxicosis due to rapid onset. T3 has a shorter half-life than levothyroxine; consider twice-daily dosing. Use with caution in elderly and patients with cardiac disease. |
| Patient Advice | Take exactly as prescribed, usually once daily. · Do not stop abruptly without consulting your doctor. · Report symptoms of hyperthyroidism: palpitations, tremor, anxiety, heat intolerance. · Store at room temperature away from moisture. |