Liothyronine (T3)

What it is

Liothyronine is synthetic T3, the active thyroid hormone that sets metabolic rate in essentially every cell. It is a legitimate treatment for hypothyroidism. Taken by someone with a normally functioning thyroid, it raises metabolic rate above where the body would set it, and the body responds by shutting down its own production.

It is prescribed for diagnosed hypothyroidism, usually where standard T4 treatment is inadequate, with thyroid function measured regularly to keep levels in range.

What’s claimed

• Accelerated fat loss through a raised metabolic rate
• The ability to eat more while dieting
• Improved energy and warmth

What the evidence shows

That T3 raises metabolic rate and drives weight loss is well-supported (Gold): the physiology is not in doubt. What the same physiology shows is that the weight lost is not selectively fat: T3 is catabolic to muscle as well, and the lean-mass cost is substantial. It also suppresses the thyroid axis, so stopping brings a period of underactive thyroid function and rapid weight regain, which is the trap the whole approach sets.

The risks

• Endocrine: suppression of the body's own thyroid production, with a hypothyroid rebound on stopping that can take a long time to resolve.
• Muscle loss: the weight lost includes meaningful lean tissue, which is the opposite of the usual goal.
• Cardiac: tachycardia, atrial fibrillation and, over time, cardiac strain; the arrhythmia risk is the main acute danger.
• Skeletal: excess thyroid hormone accelerates bone loss.
• Anxiety, tremor, heat intolerance and insomnia at supraphysiological levels.

What needs monitoring

• TSH, free T3 and free T4
• Heart rate and rhythm
• Bone density where use is prolonged: this is a prescription hormone and needs a doctor.