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. |
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 | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for TIROSINT (TIROSINT).
| Breastfeeding | Levothyroxine is secreted into breast milk in low concentrations; estimated M/P ratio 0.21–0.38. Levels are insufficient to affect infant thyroid function or cause adverse effects. Exogenous T4 is identical to endogenous; breastfeeding is considered safe with maternal dose adjustment as needed. |
| 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. |
■ FDA Black Box Warning
No FDA-required black box warning.
| Serious Effects |
["Uncorrected adrenal insufficiency","Untreated thyrotoxicosis (hyperthyroidism)","Acute myocardial infarction","Hypersensitivity to any component of the formulation"]
| 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"] |
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| 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. |