THYROLAR-0.25
Clinical safety rating: caution
Comprehensive clinical and safety monograph for THYROLAR-0.25 (THYROLAR-0.25).
Thyroid hormone (liothyronine, L-triiodothyronine or T3) binds to thyroid hormone receptors in the nucleus, altering gene transcription and protein synthesis, leading to increased metabolic rate, oxygen consumption, and thermogenesis.
| Metabolism | Hepatic metabolism primarily via deiodination to reverse T3 (rT3) and further conjugation (glucuronidation, sulfation). Minor metabolism via D1 and D2 deiodinases. |
| Excretion | Renal: ~40% as conjugated metabolites (glucuronides and sulfates); fecal: ~20% via bile; minor biliary elimination of parent drug (<5%). Total renal clearance of iodine: ~30%. |
| Half-life | Levothyroxine (T4): ~7 days; liothyronine (T3): ~1 day. Clinical context: Steady-state achieved in ~5 weeks for T4; T3 half-life shorter leads to more frequent dosing if used alone. |
| Protein binding | >99% bound to thyroxine-binding globulin (TBG), transthyretin, and albumin. T4 binding ~99.97%, T3 binding ~99.7%. |
| Volume of Distribution | T4: 0.15-0.2 L/kg; T3: 0.4-0.6 L/kg. Clinical meaning: T3 distributes more extensively into tissues due to lower protein binding, correlating with its more rapid onset and shorter duration. |
| Bioavailability | Oral: 70-80% for T4 (absorbed primarily in jejunum and ileum); 90-95% for T3. Bioavailability reduced by food, fiber, and certain drugs (e.g., iron, calcium). |
| Onset of Action | Oral (T4): 3-5 days for measurable serum T4 increase; full clinical effect in 2-3 weeks. Oral (T3): 2-4 hours for serum T3 peak; clinical effect within 24-48 hours. IV: Not applicable (no IV formulation). |
| Duration of Action | T4: 1-3 weeks after discontinuation (due to long half-life). T3: 24-72 hours. Clinical note: Once-daily dosing maintains euthyroid state due to T4's long half-life; T3 contribution requires careful monitoring to avoid fluctuation. |
Oral, 0.25 mg (1 tablet) once daily; adjust based on TSH response.
| Dosage form | TABLET |
| Renal impairment | No specific dose adjustment required for renal impairment; monitor thyroid function. |
| Liver impairment | No specific dose adjustment for Child-Pugh classes; titrate cautiously due to altered metabolism. |
| Pediatric use | Not recommended for pediatric use; safety and efficacy not established. |
| Geriatric use | Initiate at 0.125 mg orally once daily; titrate slowly based on TSH and cardiac status. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for THYROLAR-0.25 (THYROLAR-0.25).
| Breastfeeding | Minimal excretion into breast milk; normal thyroid hormone levels in milk are insufficient to cause adverse effects. M/P ratio not established for liothyronine/levothyroxine; amounts ingested are below physiological needs of infant. Compatible with breastfeeding at maternal therapeutic doses. |
| Teratogenic Risk | Thyroid hormones (T3/T4) do not cross the placenta in significant amounts; exogenous liothyronine/levothyroxine does not increase fetal malformation risk. Placental transfer of maternal TSH and thyroid hormones is regulated in pregnancy. Inadequate maternal thyroid hormone replacement increases risk of fetal neurodevelopmental deficits. No known teratogenicity at therapeutic doses. |
■ FDA Black Box Warning
No boxed warning for THYROLAR-0.25. However, all thyroid hormone preparations should not be used for treatment of obesity or weight loss; large doses may produce serious or life-threatening toxicity.
| Serious Effects |
["Untreated thyrotoxicosis (hyperthyroidism)","Uncorrected adrenal insufficiency"]
| Precautions | ["Cardiovascular effects: potential for tachycardia, palpitations, arrhythmias; caution in patients with coronary artery disease.","Endocrine: may mask signs of hyperthyroidism; need to monitor thyroid function tests.","Bone: long-term use may reduce bone mineral density.","Drug interactions: requires dose adjustment with anticoagulants, diabetes medications, and other drugs that bind to thyroid-binding proteins."] |
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| Fetal Monitoring | Monitor maternal TSH and free T4 every 4-6 weeks during pregnancy, with goal TSH 0.2-2.5 mIU/L in first trimester, 0.3-3.0 mIU/L in second/third. Fetal growth and heart rate monitoring as clinically indicated; consider umbilical cord thyroid function if maternal thyroid disease or treatment is unstable. |
| Fertility Effects | Untreated hypothyroidism is associated with ovulatory dysfunction and infertility; adequate replacement restores fertility. No direct adverse effect of thyroid hormone on fertility. |