PharmaceuticalThyroid hormoneMetabolicProhormone

Levothyroxine (T4)

Also known as T4 · Synthroid · Levoxyl · Eltroxin

Written by enwyReviewed 15 Jul 20261 sources0 comments

Levothyroxine is synthetic thyroxine (T4), the standard first-line treatment for hypothyroidism and one of the most prescribed drugs in the world. It is a prohormone converted peripherally to active T3. It is a poor off-label fat-loss agent because its effects are slow, buffered by the body's conversion regulation, and supraphysiological use carries the same thyrotoxic risks as T3.

01 Overview

Levothyroxine is synthetic T4, a prohormone with a long half-life that is deiodinated in peripheral tissues to the active hormone T3. This buffering makes it the preferred agent for stable thyroid replacement, dosed once daily and titrated to TSH. It has an extensive evidence base and a well-defined safety profile at replacement doses.

Because its conversion to T3 is regulated and its onset is slow, levothyroxine is a weak tool for deliberate metabolic acceleration compared with T3. Pushing the dose above physiological levels to force fat loss produces iatrogenic thyrotoxicosis, with cardiac strain, bone loss and lean-mass loss, and offers no advantage over simply eating in a deficit.

02 Mechanism

Synthetic thyroxine (T4) prohormone, peripherally converted to active T3, which activates nuclear thyroid receptors to set basal metabolic rate. Long half-life provides stable hormone levels.

03 Dosing

Tier	Dose	Route	Notes
Replacement	50–150 mcg/day	Oral	Typical adult replacement, titrated to TSH; full replacement ~1.6 mcg/kg/day.

04 Effects

Effect	Magnitude	Evidence
Restored metabolic rateRestores normal basal metabolic rate in hypothyroidism; does not raise it above normal at replacement doses.	normalises to euthyroid	Clinical

05 Side effects

Effect	Severity	Frequency	Evidence
Iatrogenic thyrotoxicosisOverreplacement causes hyperthyroid symptoms: palpitations, heat intolerance, weight loss, anxiety and tremor.	Moderate	With overdose	Clinical
Cardiac strainChronic overreplacement increases risk of atrial fibrillation, especially in older adults.	Severe	With chronic overdose	Clinical
Bone density lossLong-term overreplacement can accelerate bone loss, particularly in postmenopausal women.	Moderate	With prolonged excess	Clinical

06 Legal status

United States

Prescription drug

FDA-approved; among the most prescribed medications worldwide. · as of Jan 2026

07 References

Levothyroxine — clinical pharmacologyStatPearls / NCBI Bookshelf

08 Discussion0 comments

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This page is reference information, not medical advice. Doses and protocols are documented as they appear in the clinical literature and in practice — describing them is not a recommendation to use them. Countermeasures listed here are not a substitute for a physician. Legal status varies by jurisdiction and changes.