THYROLAR-0.5
Clinical safety rating: caution
Comprehensive clinical and safety monograph for THYROLAR-0.5 (THYROLAR-0.5).
Thyroid hormone replacement; L-thyroxine (T4) is converted to active triiodothyronine (T3) which binds to thyroid hormone receptors to regulate gene transcription, increasing basal metabolic rate and oxygen consumption.
| Metabolism | Hepatic metabolism via deiodination (D1, D2, D3) and glucuronidation |
| Excretion | Renal (approximately 40-50% as unchanged drug and conjugates), fecal (approximately 20-30% via biliary elimination), with the remainder metabolized and eliminated via urine and feces. |
| Half-life | For liothyronine (T3): approximately 1.5-2.5 days; for levothyroxine (T4): approximately 6-7 days. In hyperthyroidism, half-life may be shortened; in hypothyroidism, prolonged. |
| Protein binding | Greater than 99% bound, primarily to thyroxine-binding globulin (TBG), also to transthyretin and albumin. |
| Volume of Distribution | 0.2-0.4 L/kg for T4; 0.4-0.6 L/kg for T3, reflecting extensive tissue distribution with T3 having higher apparent Vd. |
| Bioavailability | Oral: approximately 80-95% for T4; approximately 95% for T3. Absorption is reduced by food, especially for T4. |
| Onset of Action | Oral: liothyronine (T3) component: onset of action within 6-12 hours; levothyroxine (T4) component: onset of action within 3-5 days. |
| Duration of Action | Liothyronine (T3): duration of action approximately 2-3 days; levothyroxine (T4): duration of action approximately 10-14 days. Clinical effect persists for weeks after discontinuation due to T4 reservoir. |
Initial dose 0.5 tablets (30 mg T4/7.5 mg T3) orally once daily, titrated every 2-4 weeks based on TSH, free T4, and free T3 levels; usual maintenance 0.5-2 tablets (30-120 mg T4/7.5-30 mg T3) once daily.
| Dosage form | TABLET |
| Renal impairment | No specific dose adjustment required for GFR >=15 mL/min. For GFR <15 mL/min or dialysis, start with 0.25 tablets (15 mg T4/3.75 mg T3) once daily and titrate slowly due to reduced clearance of T3. |
| Liver impairment | Child-Pugh Class A: No adjustment. Child-Pugh Class B or C: Start with 0.25 tablets (15 mg T4/3.75 mg T3) once daily and titrate cautiously every 4-6 weeks due to impaired conversion of T4 to T3. |
| Pediatric use | Dose based on weight: 0.5-1 mg T4/kg/day and 0.125-0.25 mg T3/kg/day orally once daily, adjusted based on TSH and free T4 levels. Titrate in increments of 0.125-0.25 tablets at 4-week intervals. |
| Geriatric use | Start with 0.25 tablets (15 mg T4/3.75 mg T3) once daily. Titrate by 0.125 tablets every 4-6 weeks due to increased sensitivity to T3 and higher risk of cardiac side effects. Monitor closely for arrhythmias and symptoms of hyperthyroidism. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for THYROLAR-0.5 (THYROLAR-0.5).
| Breastfeeding | Minimal excretion into breast milk; amount is insufficient to affect infant thyroid function or cause adverse effects. No M/P ratio is established for liothyronine; levothyroxine M/P ratio approximately 0.4. Compatible with breastfeeding. |
| Teratogenic Risk | Pregnancy Category A. Thyroid hormones do not cross the placenta in significant amounts; however, maternal hypothyroidism during pregnancy is associated with fetal risks. Untreated maternal hypothyroidism may increase risk of miscarriage, preterm birth, and neurodevelopmental deficits. Exogenous levothyroxine (T4) is considered safe. Liothyronine (T3) at high doses may theoretically affect fetal development, but no increased risk of congenital anomalies has been reported with replacement doses. |
■ FDA Black Box Warning
Not indicated for treatment of obesity or weight loss in euthyroid patients; may cause serious or life-threatening toxicity, especially when used in combination with sympathomimetic amines.
| Serious Effects |
["Untreated thyrotoxicosis","Uncorrected adrenal insufficiency","Acute myocardial infarction","Hypersensitivity to any component"]
| Precautions | ["Cardiac toxicity in overdosage; risk of arrhythmias, angina, myocardial infarction","Pre-existing cardiovascular disease; use reduced initial dose","Adrenal insufficiency: may precipitate crisis; correct before initiating therapy","Thyrotoxicosis: avoid use","Concomitant use with oral anticoagulants requires monitoring"] |
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| Fetal Monitoring | Monitor maternal thyroid function tests (TSH, free T4, free T3) every 4-6 weeks during pregnancy and postpartum; adjust dose to maintain TSH in trimester-specific reference range. Assess fetal heart rate and growth by ultrasound if maternal thyroid disease is poorly controlled. Monitor for maternal tachycardia, arrhythmias, or symptoms of hyperthyroidism. |
| Fertility Effects | Untreated hypothyroidism can cause ovulatory dysfunction, anovulation, and infertility. Thyroid hormone replacement restores euthyroid state, improving fertility outcomes. No direct adverse effects on fertility from the medication itself. |