T4 · Synthroid · Levothroid · Levothyroxine Sodium
Levothyroxine sodium (T4) is the synthetic form of thyroxine, the primary storage hormone produced by the thyroid gland. It is converted peripherally in the liver, kidneys, and other tissues to the biologically active form, triiodothyronine (T3), via the deiodinase enzymes. Levothyroxine is one of the most prescribed medications in the world and is the first-line treatment for primary hypothyroidism. It is FDA-approved under brand names including Synthroid, Levoxyl, Tirosint, Euthyrox, and Unithroid.
Take in the morning on an empty stomach, 30-60 minutes before breakfast, calcium, iron supplements, or antacids, as these dramatically reduce absorption. Starting dose is typically 25-50 mcg/day for adults without cardiac disease, with 12.5 mcg increments every 4-6 weeks based on TSH response. Do not expect to feel a change for the first 1-2 weeks - T4 takes 4-6 weeks to reach new steady state.
Start here. Every protocol below is ordered lowest-first. Begin at the smallest effective dose, hold about a week to assess tolerance, and step up only if needed.
Levothyroxine is identical to endogenous T4. It serves as the circulating storage form of thyroid hormone and is converted to the active T3 by peripheral deiodinase enzymes (predominantly type 1 and type 2 deiodinases) in the liver, kidneys, pituitary, thyroid, skeletal muscle, and brain. The resulting T3 binds nuclear thyroid hormone receptors (TR-alpha and TR-beta), regulating transcription of genes controlling basal metabolic rate, oxygen consumption, protein turnover, carbohydrate and lipid metabolism, and thermogenesis.
The long plasma half-life (approximately 7 days) reflects high-affinity binding to thyroxine-binding globulin (TBG), transthyretin, and albumin. This binding pool buffers daily fluctuations and produces the stable circulating levels that make T4 the preferred replacement hormone. A dose change requires 4-6 weeks (five half-lives) to reach new steady state, which is why TSH should not be re-measured earlier than six weeks after a dose adjustment.
Food, calcium, iron, aluminum-containing antacids, bile acid sequestrants, and soy proteins reduce absorption significantly - hence the 30-60 minute empty-stomach dosing convention. Peripheral T4-to-T3 conversion is reduced by glucocorticoids, propranolol at high doses, amiodarone, and certain illnesses (for example severe systemic illness, 'low T3 syndrome').
First 1-2 weeks: minimal perceptible change. Plasma T4 is climbing toward new steady state but peripheral T3 levels lag.
Weeks 3-6: clinical improvement in fatigue, cold intolerance, constipation, dry skin, and mood. TSH begins to drop. Recheck TSH at week 6 and adjust dose accordingly.
Months 2-4: full steady state and symptomatic benefit. Subsequent dose changes take another 6 weeks to equilibrate. Maintenance dosing is typically lifelong in primary hypothyroidism.
Each reference below was re-fetched from its PubMed record, and the quoted sentence was confirmed to appear in that abstract and to support the claim it sits under. Follow any of them to read the source.
Exact-molecule evidence boundary: the live abstract reports the quoted finding for T4 (Levothyroxine / Synthroid); no broader inference is made.
Many studies have found clinical and metabolic alterations in subclinical hypothyroidism, however, there are disagreements about the benefits of levothyroxine therapy.
Evidence scope. Exact molecule wording appears in the quoted live sentence. Only the population/model, formulation, route, and outcome expressly stated there are supported.
Claims we could not support
No abstract we checked supports these for this molecule at this route. That is not the same as saying they are false — it marks where the evidence is missing.
Dose/route: 25mcg; Oral; 1x daily. No selected live abstract sentence verified this complete regimen.
Mechanism as written in source JSON. No selected live abstract sentence verified the complete mechanism at the card formulation/route and relevant population/model.
Primary safety claim as written in source JSON. No selected live abstract sentence verified the complete safety claim for this exact formulation and route.
Approximately 1 week
(absorption); days to weeks for clinical effect
To full steady state after dose change
Oral (highly food- and supplement-interaction dependent)
Per dose - supports once daily dosing with stable blood levels
Hepatic deiodination (T4 -> T3 or T4 -> rT3) and conjugation. Biliary and renal excretion of metabolites.
Storage. Store at controlled room temperature (20-25°C / 68-77°F). Protect from light and moisture. Keep in original container. Shelf life typically 2-3 years from manufacture date. Moisture can degrade potency - do not transfer to humid pill organizers long-term. Keep out of reach of children.
FDA-approved under brand names including Synthroid (AbbVie), Levoxyl (Pfizer), Tirosint (IBSA), Euthyrox (Merck KGaA), and Unithroid (Jerome Stevens). Generic levothyroxine is widely available. Prescription-only in the United States; not a controlled substance. Not prohibited by WADA - thyroid hormones are not on the current WADA prohibited list, though monitored in athletes. Internationally available under various brand names.
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