Levothyroxine (T4)
Also known as: T4, Euthyrox, L-Thyroxin, Synthroid, Thyroxine
The storage form of thyroid hormone, used medically to replace what the thyroid does not make — frequently confused with T3 as a fat loss tool, which is not what it is or how it behaves.
Substance family
They increase metabolic rate or mobilise fat through adrenergic and thermogenic mechanisms. Cardiac load is the shared limiting factor, and it adds up when they are combined.
Effects on
Which outcomes this substance acts on — descriptive, not a recommendation.
Strain profile
Where this substance is most likely to hit — a rough orientation, individually dependent on dose, duration and predisposition.
This substance causes
Dosing & protocols by goal
Rough orientation ranges as reported in harm-reduction literature — not an individual treatment plan. Doses are individual and open-ended; frequency depends on the ester or half-life.
Thyroid replacement
Medical treatmentDose
Individually titrated to TSH, physician-set
Frequency
1× / day, fasted
Cycle length
Ongoing, as prescribed
Dosed against a lab value, not a goal. Adjustments are made every 6–8 weeks because the level takes that long to settle.
Restarting after a T3 course
RecoveryDose
Physician-set, if needed at all
Frequency
1× / day
Cycle length
Until the axis recovers
Whether this is needed is a blood test question, not a default. Many recover without anything.
Side effect & countermeasure
Typical problems and what you can do to reduce harm — at a glance. Does not replace medical monitoring.
Problem
Taken for fat loss, nothing happens
Countermeasure
Expected. In someone with a working thyroid, added T4 largely suppresses their own production and the net change is small. T3 is the compound with the acute effect — and its own consequences.
Problem
Own thyroid suppressed after a T3 course
Countermeasure
Test TSH, fT3 and fT4 before assuming anything. Recovery is usually spontaneous over weeks; replacement is a medical decision based on those values.
Problem
Heart rate up, tremor, poor sleep
Countermeasure
Signs of too much. Because the half-lifehalf-lifeHow long it takes for half the substance to leave the body — it determines dosing frequency, not duration of effect. is a week, changes take a fortnight to show — reduce and wait rather than adjusting again after three days.
Overview
Levothyroxine is synthetic T4, the storage form of thyroid hormone. The body converts it to T3, the active form, as needed.
It appears on this site mostly because of a confusion: T3 is used in this context for fat loss, T4 is not, and the two get discussed as though they were interchangeable versions of the same thing.
Mechanism of Action
T4 is a reservoir. It circulates bound and largely inactive, and enzymes convert it to T3 where and when it is needed. That conversion is regulated — which is precisely why taking T4 to accelerate metabolism works poorly in someone whose thyroid is functioning.
Add T4, and TSH falls, and your own production falls with it. The total ends up close to where it started.
Why it is here
Two situations make it relevant.
After a T3 course. Exogenous T3 suppresses TSH, and with it the thyroid's own output. Coming off, some people find themselves temporarily hypothyroid — flat, cold, tired, with a stalled metabolism at exactly the moment they were counting on it. Most recover on their own over weeks. Whether replacement helps is a question for TSH, fT3 and fT4, not for how you feel.
Genuine hypothyroidism, which is more common than assumed and produces symptoms — fatigue, weight gain, cold intolerance — that get blamed on training or diet for years.
The half-life changes how you use it
Seven days is long. It means a dose change takes around six weeks to fully show up in blood work, and that any adjustment made after a few days is being made blind.
This is the opposite of T3, where effects arrive within days. Applying T3 habits to T4 produces a lot of unnecessary oscillation.
Taking it correctly
Absorption is genuinely fussy, and this is where most of the "it does not work" comes from:
- Fasted, 30–60 minutes before food
- Not with coffee — it measurably reduces absorption
- Calcium, iron and magnesium at least four hours apart, which matters if you supplement any of them
Side Effects & Risks
- Raised resting heart rate, palpitations, tremor when overdosed
- Sleep disruption and anxiety
- Bone loss with long-term over-replacement
- Suppression of your own thyroid axis while taking it
Blood Work & Monitoring
- TSH — the primary marker; it is what dosing is set against
- fT3 and fT4 — for the full picture, especially after a T3 course
- Recheck 6–8 weeks after any change, not sooner
Harm-Reduction Notes
- This is replacement, not a fat loss tool — treating it as one mostly suppresses your own production
- Dose against a lab value, never against the scale
- Change one thing, wait six weeks, retest
- After a T3 course, test before treating. Most axes recover on their own, and starting replacement unnecessarily creates the dependency people feared
Legal status by region
Regulation differs considerably between jurisdictions and changes over time. This is a rough orientation, not legal advice — check the rules that apply where you are.
| Region | Regulatory status |
|---|---|
| United States | Prescription-only medicine |
| EU / UK | Prescription-only medicine |
| Canada / Australia | Prescription-only medicine |
| Parts of Asia, Latin America & Middle East | Pharmacy availability varies |
| Competitive sport | Check the current WADA list |