LIOTHYRONINE SODIUM
Clinical safety rating: safe
Human studies have proved safety
Liothyronine is a synthetic form of triiodothyronine (T3), the active thyroid hormone. It binds to thyroid hormone receptors in the nucleus, modulating gene transcription and increasing basal metabolic rate, oxygen consumption, and thermogenesis. It enhances carbohydrate and lipid metabolism, and promotes normal growth and development.
| Metabolism | Liothyronine is primarily metabolized in the liver via deiodination (type I and II deiodinases) to form reverse T3 (rT3) and other inactive metabolites, as well as glucuronidation and sulfation conjugation pathways. A small amount is excreted unchanged in urine. |
| Excretion | Primarily renal (approximately 50% as unchanged drug and metabolites); minor biliary/fecal elimination. |
| Half-life | Approximately 1-2 days in euthyroid patients; shorter in hyperthyroidism, prolonged in hypothyroidism. Clinical context: requires monitoring of thyroid function tests for dose adjustment. |
| Protein binding | >99% bound primarily to thyroxine-binding globulin (TBG), transthyretin, and albumin. |
| Volume of Distribution | Approximately 0.4-0.5 L/kg, indicating distribution into total body water with some tissue binding. |
| Bioavailability | Oral: 80-95% (highly absorbed); intravenous: 100%. |
| Onset of Action | Oral: 2-4 hours; Intravenous: within minutes (peak effect in 2-4 hours). |
| Duration of Action | Oral: 24-48 hours; Intravenous: 24-72 hours. Clinical note: due to short half-life, multiple daily doses may be required. |
| Molecular Weight | 672.97 |
25-75 mcg orally once daily; initial dose 25 mcg daily, titrate by 12.5-25 mcg increments every 1-2 weeks based on response.
| Dosage form | TABLET |
| Renal impairment | No specific dose adjustment provided; use with caution in severe renal impairment due to potential accumulation. |
| Liver impairment | Child-Pugh A: no adjustment; Child-Pugh B/C: reduce starting dose by 50% and titrate cautiously. |
| Pediatric use | Initial dose 5-20 mcg orally once daily, titrate by 5 mcg increments every 2 weeks; maintenance based on weight: 2-4 mcg/kg/day. |
| Geriatric use | Start at 12.5-25 mcg orally once daily; titrate by 12.5 mcg increments every 2-4 weeks; lower doses due to increased sensitivity. |
| 1st trimester | Liothyronine crosses the placenta; use only if clearly needed. Maintain euthyroid state as hypothyroidism is harmful. Dose adjustments may be needed. |
| 2nd trimester | Similar to T1; monitor thyroid function and adjust dose to maintain maternal euthyroidism. |
| 3rd trimester | Similar to T1/T2; postpartum dose may need reduction as pregnancy-induced changes resolve. |
Clinical note
Many drugs can decrease absorption (eg calcium iron) Can cause cardiac ischemia and arrhythmias in overdose.
| Placental transfer | Liothyronine (T3) crosses the placenta; transfer is similar to endogenous T3. Placental metabolism may reduce fetal exposure. |
| Breastfeeding | Liothyronine is excreted into breast milk in low amounts, but doses used for maternal hypothyroidism are unlikely to affect the infant. Monitor infant thyroid function if mother is on high doses. |
■ FDA Black Box Warning
No FDA black box warning.
| Common Effects | Palpitations |
| Serious Effects |
Untreated thyrotoxicosisAcute myocardial infarctionUncontrolled adrenal insufficiency
| Precautions | Cardiac toxicity: increased heart rate, angina, arrhythmias, especially in elderly or with CV disease, Thyrotoxicosis: symptoms of hyperthyroidism (tachycardia, tremor, weight loss) require dose reduction, Drug interactions: warfarin (increased anticoagulant effect), oral hypoglycemics (increased glucose), digitalis (decreased efficacy), Use with caution in patients with adrenal insufficiency (may precipitate adrenal crisis), Pregnancy: crosses placenta, monitor maternal thyroid function |
| Food/Dietary | Avoid large amounts of goitrogenic foods (e.g., cabbage, kale, brussels sprouts, turnips, soy) as they may interfere with thyroid function. High-fiber diets can reduce absorption. Separate doses from calcium or iron supplements, antacids, or sucralfate by at least 4 hours. |
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| Lactation Rating | L2 (Safer) |
| Teratogenic Risk | Liothyronine is a thyroid hormone that does not cross the placenta in significant amounts; therefore, it is not associated with teratogenicity. Maternal hypothyroidism itself increases risk of fetal loss, preterm delivery, and neurodevelopmental deficits. Adequate treatment is essential. No trimester-specific risk. |
| Fetal Monitoring | Monitor maternal thyroid function tests (TSH, free T4, T3) every 4-6 weeks during pregnancy and postpartum. Fetal monitoring: ultrasound for growth and heart rate if clinical signs of thyroid dysfunction or if mother has underlying thyroid disease. |
| Fertility Effects | Untreated hypothyroidism can cause anovulation and infertility. Liothyronine treatment restores euthyroid state, improving fertility outcomes. No direct adverse effects on fertility at therapeutic doses. |
| Clinical Pearls | Liothyronine (T3) has a rapid onset and short half-life (~1 day), making it useful for thyroid suppression testing and myxedema coma, but less preferred for chronic hypothyroidism due to fluctuating levels. Monitor thyroid function tests closely; T3 therapy will suppress TSH and may elevate FT3 disproportionately to FT4. Use cautiously in elderly or cardiac patients to avoid tachyarrhythmias. For myxedema coma, give IV liothyronine; oral bioavailability is >90%. |
| Patient Advice | Take exactly as prescribed; do not stop abruptly. · Monitor for signs of hyperthyroidism: rapid heartbeat, chest pain, sweating, nervousness, or weight loss. · Report any chest pain or palpitations immediately. · Take on empty stomach with water, 30 minutes before breakfast for consistent absorption. · Do not switch brands or formulations without consulting your provider. · Keep regular appointments for thyroid function tests. · Inform all healthcare providers you are taking this medication. · If miss a dose, skip it unless close to next dose; do not double. |