Clinical safety rating: safe
Human studies have proved safety
Levothyroxine is a synthetic form of thyroxine (T4) that is converted to triiodothyronine (T3) in peripheral tissues. T3 binds to thyroid hormone receptors in the nucleus, regulating gene transcription and increasing metabolic rate, protein synthesis, and oxygen consumption.
| Metabolism | Converted to active triiodothyronine (T3) primarily in the liver and kidneys via deiodination by iodothyronine deiodinases (D1 and D2). Both T4 and T3 undergo glucuronidation and sulfation in the liver. Approximately 20% of T4 is metabolized via conjugation and excretion in bile. |
| Excretion | Primarily renal (approximately 50% as unchanged drug and metabolites, mainly glucuronide and sulfate conjugates); minor biliary/fecal excretion (<20%). |
| Half-life | 7 days (euthyroid); prolonged in hyperthyroidism (3-4 days) and shortened in hypothyroidism (9-10 days); clinical effects persist for weeks after discontinuation due to slow elimination. |
| Protein binding | >99% bound to thyroxine-binding globulin (TBG), transthyretin, and albumin; unbound fraction <0.05%. |
| Volume of Distribution | 0.2-0.3 L/kg (reflects extensive extracellular distribution but limited intracellular penetration; higher Vd in hyperthyroidism). |
| Bioavailability | Oral: 40-80% (absorption decreased by food, fiber, iron, calcium, and malabsorption syndromes); IV: 100%. Note: Absorption is variable; consistent brand and fasting administration recommended. |
| Onset of Action | Oral: 3-5 days (full therapeutic effect in 2-4 weeks); IV: within 6-8 hours for myxedema coma (due to rapid conversion to T3). |
| Duration of Action | Oral: 2-3 weeks (hormonal effects persist after stopping due to long half-life); dosing once daily maintains steady state. |
| Molecular Weight | 776.87 |
1.6 mcg/kg orally once daily, adjusted based on TSH levels; typical adult dose 50-200 mcg/day.
| Renal impairment | No dose adjustment required for renal impairment as levothyroxine is primarily hepatically metabolized. |
| Liver impairment | No specific Child-Pugh based dose adjustments; monitor TSH levels and adjust accordingly in hepatic impairment. |
| Pediatric use | Neonates: 10-15 mcg/kg/day orally; Infants: 5-10 mcg/kg/day; Children: 2-5 mcg/kg/day; start at lower end and titrate to normal TSH/fT4. |
| Geriatric use | Start at lower dose (25-50 mcg/day) and titrate slowly by 12.5-25 mcg increments every 4-6 weeks to avoid cardiac stress; target TSH higher if indicated. |
| 1st trimester | Levothyroxine is essential for maternal and fetal thyroid function; hypothyroidism must be corrected. Dose adjustments may be required due to increased T4 clearance. No known teratogenicity. |
| 2nd trimester | Continue therapy; increased T4 requirements may persist. Monitor thyroid function tests every 4-6 weeks. |
| 3rd trimester | Maintain euthyroid state; dose adjustments may be needed. No known fetal harm. |
Clinical note
Essential in pregnancy. Maternal thyroid hormone is critical for fetal brain development, particularly in T1 before the fetal thyroid is functional (~12 weeks). Untreated hypothyroidism causes irreversible fetal neurodevelopmental impairment. Dose requirements increase by approximately 30–50% in T1 due to increased TBG, placental T4 metabolism, and hCG-driven thyroid stimulation. TSH targets in pregnancy: T1 <2.5 mIU/L, T2 and T3 <3.0 mIU/L (or institution-specific).
| Placental transfer | Limited placental transfer;<0.01% of maternal dose crosses placenta. Endogenous thyroid hormones cross but levothyroxine is a synthetic T4 that behaves similarly. |
■ FDA Black Box Warning
Not indicated for weight loss or obesity treatment; serious cardiovascular toxicity or death may occur, especially when used in combination with sympathomimetic amines or for weight reduction.
| Serious Effects |
Untreated thyrotoxicosisAcute myocardial infarctionUncorrected adrenal insufficiency
| Precautions | Cardiovascular effects: May increase heart rate and contractility, risk of arrhythmias and myocardial ischemia, especially in patients with coronary artery disease., Thyrotoxic crisis: Overdose can cause thyrotoxicosis; symptoms include tachycardia, anxiety, chest pain, and cardiac arrest., Adrenal insufficiency: Exogenous thyroid hormone may precipitate adrenal crisis in undiagnosed adrenal insufficiency; treat adrenal insufficiency before starting levothyroxine., Osteoporosis: Long-term TSH suppression carries risk of decreased bone mineral density., Concomitant anticoagulation: May increase anticoagulant effect of warfarin. |
| Food/Dietary | High-fiber foods, soy products, walnuts, cottonseed meal, and calcium-fortified juices can reduce levothyroxine absorption. Grapefruit juice may also affect absorption. Iron and calcium supplements, antacids (aluminum/magnesium), sucralfate, and orlistat should be separated by at least 4 hours. Caffeine can interfere if taken simultaneously. A consistent dietary routine is advised. |
Loading safety data…
| Breastfeeding |
| Levothyroxine is excreted into breast milk in minimal amounts, not sufficient to affect infant thyroid function or interfere with neonatal screening. It is considered compatible with breastfeeding. |
| Lactation Rating | L1 - Safe |
| Teratogenic Risk | No known teratogenic risk. Fetal thyroid development dependent on maternal hormone levels; hypothyroidism associated with adverse outcomes. FDA category A. |
| Fetal Monitoring | Maternal: TSH, free T4 every 4-6 weeks during first half, then every 6-8 weeks. Fetal: Ultrasound for growth and heart rate; consider cordocentesis for severe maternal thyroid disease. |
| Fertility Effects | Hypothyroidism can cause anovulatory cycles and infertility; euthyroid state essential for conception. Levothyroxine replacement restores fertility. |
| Clinical Pearls | Levothyroxine should be taken on an empty stomach, at least 30-60 minutes before breakfast, to ensure consistent absorption. Avoid co-administration with calcium or iron supplements, antacids, or bile acid sequestrants; separate by at least 4 hours. TSH levels should be checked 6-8 weeks after dose changes. In patients with coronary artery disease, start with a low dose (12.5-25 mcg daily) to avoid precipitating angina. Thyroid function tests can be altered by pregnancy, estrogen therapy, and severe illness. |
| Patient Advice | Take this medication at the same time each day, preferably in the morning on an empty stomach. · Do not stop taking this medication without consulting your doctor; it is usually taken for life. · Report symptoms of over- or under-treatment (e.g., palpitations, weight changes, fatigue). · If you miss a dose, take it as soon as you remember, unless it's almost time for the next dose. Do not double the dose. · Inform all healthcare providers that you are taking levothyroxine. · If you become pregnant, notify your doctor; dosage may need to be adjusted. |