Liothyronine (T3)

Overview

Liothyronine is synthetic T3, the active form of thyroid hormone. Medically it treats an underactive thyroid; the use described here is off-label and pursues the opposite direction — raising metabolic rate above the normal level.

It works, and it works considerably more reliably than clenbuterol. That is exactly why it is the more consequential substance: T3 does not intervene in a receptor system but in one of the body's central control loops, and it costs muscle alongside fat.

Mechanism of Action

The thyroid produces mainly T4, a storage form converted in tissue to the active T3. T3 regulates basal metabolic rate — how much energy the body expends at rest.

The control loop runs through TSH: the pituitary measures thyroid hormone levels and adjusts its signal accordingly. External T3 registers as a sufficient level, TSH falls, and the thyroid reduces its own production. This is not a side effect but an unavoidable consequence, and it is the reason the substance cannot simply be stopped from one day to the next.

A deficit itself already lowers T3 — the body reduces energy expenditure when eating less. Low-dose T3 essentially replaces that reduction, which is the rationale behind the twelve-and-a-half to twenty-five microgram approach. Higher doses push into genuine surplus, with correspondingly stronger effects in both directions.

The point that matters most in this context: T3 raises the turnover of all substrates, not just fat. Protein is broken down alongside it. In a deficit without adequate protein, training stimulus and androgen base, a substantial share of what is lost is muscle.

Typical Context

T3 is used in diet phases over four to six weeks, tapering both up and down. Doses in the low range are the more defensible choice, since they largely replace what the deficit removes rather than creating a hyperthyroid state.

Combination with clenbuterol is widespread. Both raise cardiac output, and the effects add — this is where the practical risk of the combination lies, not in either substance alone.

The end of a course requires as much planning as the start. Abrupt discontinuation leaves a window in which the body's own production has not yet resumed and the external supply is gone. What follows is temporary hypothyroid function with lethargy, cold intolerance and rapid weight regain — a pattern often blamed on the diet rather than on the taper that never happened.

Side Effects & Risks

• Suppression of the body's own thyroid function
• Considerable loss of lean mass alongside fat
• Elevated resting heart rate, palpitations, arrhythmia
• Muscle weakness, fatigue with excessive dosing
• Hair loss
• Sweating, heat intolerance, restlessness
• Bone density loss with prolonged use
• Rebound lethargy and weight regain after abrupt discontinuation

Blood Work & Monitoring

• TSH, fT3, fT4 — before, during and 6–8 weeks after; the essential set here
• Resting heart rate — measured daily
• Body composition — scale weight hides how much of the loss is muscle
• Blood pressure — measured at home
• Lipid profile (LDL, HDL, triglycerides)

Harm-Reduction Notes

• Taper both up and down; abrupt discontinuation leaves a genuine hormonal gap
• Low doses (12.5–25 µg) replace what the deficit removes and are the more defensible approach
• Protein at roughly 2.5 g per kg and resistance training throughout — T3 does not spare muscle
• Not without a testosterone base at higher doses; muscle loss is otherwise substantial
• Measure thyroid values before starting, not only afterwards
• Combined with clenbuterol: low doses of both, daily heart-rate checks
• Muscle weakness and fatigue mean the dose is too high, not that it is working