TIROSINT
Clinical safety rating: caution
Comprehensive clinical and safety monograph for TIROSINT (TIROSINT).
Tirosint is a synthetic form of levothyroxine (T4), which is converted to triiodothyronine (T3) in peripheral tissues. T3 binds to thyroid hormone receptors in the nucleus, modulating gene transcription to increase metabolic rate, protein synthesis, and oxygen consumption.
| Metabolism | Primarily hepatic via deiodination (type I and II deiodinases) to active T3 and reverse T3; also conjugated with glucuronides and sulfates and excreted in bile and urine. |
| Excretion | Renal (approximately 30-40% as unchanged drug and metabolites, primarily glucuronide and sulfate conjugates); fecal (approximately 20-30% via bile); total clearance is low (~0.05 L/hr/kg). |
| Half-life | Terminal half-life approximately 7 days in euthyroid individuals; prolonged in hypothyroidism (up to 9-10 days) and shortened in hyperthyroidism (3-4 days). Clinical context: steady-state reached in 4-6 weeks; dosage adjustments require 6-8 weeks for full effect. |
| Protein binding | >99% bound to thyroxine-binding globulin (TBG, ~70%), transthyretin (TTR, ~20%), and albumin (~10%). Binding capacity affected by pregnancy, estrogens, and liver disease. |
| Volume of Distribution | Vd approximately 0.12-0.24 L/kg, reflecting distribution into extracellular fluid and limited intracellular penetration. Clinical meaning: low Vd indicates primarily plasma and interstitial binding; not extensively stored in tissues. |
| Bioavailability | Oral: approximately 80% (range 70-90%). Absorption is enhanced by fasting state and reduced by food (especially high fiber). IV: 100%. |
| Onset of Action | Oral: 3-5 days for measurable rise in serum T4; full metabolic effects (e.g., BMR increase) in 10-14 days. IV: more rapid onset, but not commonly used. |
| Duration of Action | Duration: Approximately 2-3 weeks after a single dose, reflecting slow clearance. Clinical note: once-daily dosing maintains stable levels; missed doses can be taken within 12 hours without significant fluctuation. |
| Molecular Weight | 776.87 |
Initial dose 1.6 mcg/kg orally once daily, adjusted based on TSH levels. Typical maintenance dose 50-200 mcg/day.
| Dosage form | CAPSULE |
| Renal impairment | No dose adjustment required for renal impairment. |
| Liver impairment | No dose adjustment required for hepatic impairment. |
| Pediatric use | Initial dose 10-15 mcg/kg/day orally once daily; adjust based on TSH and free T4 levels. |
| Geriatric use | Start with lower initial dose (12.5-25 mcg/day) and titrate slowly every 4-6 weeks due to increased sensitivity. |
| 1st trimester | Thyroid hormones are essential for fetal neurodevelopment; levothyroxine crosses the placenta in limited amounts. Hypothyroidism in pregnancy requires dose adjustment; monitor TSH levels. No evidence of teratogenicity. |
| 2nd trimester | Continue therapy; increased maternal thyroid hormone requirements may necessitate dose adjustments. Monitor TSH every 4-6 weeks. |
| 3rd trimester | Continue therapy; monitor TSH peripartum. Levothyroxine is safe and necessary for maternal and fetal health. |
Clinical note
Comprehensive clinical and safety monograph for TIROSINT (TIROSINT).
| Placental transfer | Limited placental transfer; levothyroxine crosses the placenta in small amounts, but maternal thyroid hormones are critical for fetal development. Endogenous thyroid hormones are regulated. |
| Breastfeeding | Levothyroxine is excreted into breast milk in low concentrations that are not expected to cause adverse effects in the infant. Monitor infant thyroid function if high doses are used. Considered compatible with breastfeeding. |
■ FDA Black Box Warning
No FDA-required black box warning.
| Serious Effects |
Uncorrected adrenal insufficiencyUntreated thyrotoxicosisHypersensitivity to levothyroxine or any component
| Precautions | Cardiac toxicity (tachycardia, arrhythmias) in patients with cardiovascular disease, Adrenal insufficiency may be unmasked or precipitated in patients with pituitary or hypothalamic disorders, Chronic hypothyroidism can lead to myxedema coma if therapy is not initiated carefully |
| Food/Dietary | Grapefruit juice, high-fiber foods, soy products, walnuts, and cottonseed meal may reduce absorption. Take TIROSINT on an empty stomach and avoid these foods for at least 30-60 minutes after dosing. |
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| Lactation Rating | L1 (Safe) |
| Teratogenic Risk | Untreated maternal hypothyroidism increases risk of fetal loss, preterm delivery, and neurodevelopmental deficits. Levothyroxine (T4) crosses placenta minimally; fetal thyroid function is autonomous by 12 weeks. No known teratogenicity at therapeutic doses. Trimester-specific risks: First trimester: essential for fetal brain development. Second/third: fetal thyroid compensates partially; maternal hypothyroidism still risks fetal growth restriction and cognitive impairment. |
| Fetal Monitoring | Monitor maternal TSH every 4–6 weeks during pregnancy, with goal trimester-specific (first: 0.2–2.5 mIU/L, second: 0.3–3.0 mIU/L, third: 0.3–3.5 mIU/L) or as per guidelines. Monitor fetal growth and development via ultrasound. Assess neonatal thyroid function at birth if maternal thyroid disease or high dose. |
| Fertility Effects | Untreated hypothyroidism can cause menstrual irregularities, anovulation, and infertility. Euthyroidism restoration with levothyroxine typically normalizes fertility. No direct adverse fertility effects from drug itself. |
| Clinical Pearls |
| TIROSINT is a synthetic T4 (levothyroxine sodium) used for hypothyroidism. Administer on an empty stomach, at least 30-60 minutes before breakfast, with a full glass of water. Avoid simultaneous administration with calcium carbonate, iron, soy, or high-fiber foods. Monitor TSH levels 6-8 weeks after dose changes. Dose adjustments may be needed with weight changes, pregnancy, or concurrent medications like estrogen or anticonvulsants. |
| Patient Advice | Take TIROSINT exactly as prescribed, usually once daily on an empty stomach. · Do not stop or change dose without consulting your doctor. · Wait at least 4 hours after taking TIROSINT before taking antacids, iron, or calcium supplements. · Inform your doctor if you become pregnant or are breastfeeding. · Report symptoms of hyperthyroidism (rapid heartbeat, anxiety, weight loss) or hypothyroidism (fatigue, cold intolerance, constipation). · Store at room temperature, away from light and moisture. |