Levothyroxine: Difference between revisions
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== References == | == References == | ||
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[[Category:Thyroid hormones]] | |||
[[Category:Hormone replacement]] | |||
Latest revision as of 10:43, 23 May 2026
Levothyroxine
Synthroid, Levoxyl, Tirosint, Unithroid, Euthyrox
Experience
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Summary
Classes
Common uses
Hypothyroidism0, Goiter0, TSH suppression in thyroid cancer0
Pharmacy
Starting dose
1.6 mcg/kg/d in young healthy adults; 25-50 mcg/d in elderly or cardiac disease, titrated by TSH at 6-8 weeks
Preparations
25, 50, 75, 88, 100, 112, 125, 137, 150, 175, 200, 300 mcg tablets; oral capsule and IV/IM also available
US FDA Max
No fixed maximum; titrated to TSH target
Pharmacology
Routes
Oral, IV
Onset
TSH normalization 4-8 weeks; symptomatic improvement weeks to months
Duration
Steady-state at 4-6 weeks
Half-life
~7 days (euthyroid); longer in hypothyroidism (~9-10 days), shorter in hyperthyroidism[1]
Bioavailability
40-80% (oral); reduced by food, calcium, iron, PPIs, fiber; take fasting with water[1]
Pregnancy
First-line in pregnancy; dose typically increased 25-30% due to estrogen-driven rise in TBG and fetal demand. Lactation safe at physiologic doses.[citation needed]
Legal status
Rx-only in US
Purported mechanism
Synthetic T4 (thyroxine); peripherally deiodinated to T3 (triiodothyronine), the active hormone. Levothyroxine binds nuclear thyroid hormone receptors to drive transcription of genes governing basal metabolic rate, thermogenesis, cardiac inotropy, lipid handling, and bone turnover.0 Narrow therapeutic index; brand-to-generic switches can shift TSH and require re-titration[1].
References
- ↑ 1.0 1.1 1.2 FDA Prescribing Information, Synthroid (levothyroxine sodium), AbbVie, current revision. https://www.accessdata.fda.gov/drugsatfda_docs/label/2017/021402s032lbl.pdf