T3 (liothyronine, sold as Cytomel) is the active thyroid hormone. It raises metabolic rate, which is why it is used in contest preparation. Excess T3 also breaks down muscle, strains the heart and suppresses your own thyroid, so it carries real risks outside medical use.
- In Canada it is a prescription drug for an underactive thyroid.
- Too much T3 causes a fast heart rate, palpitations, anxiety and muscle loss.
- Natural thyroid output is suppressed during use and needs time to recover.
Thyroid hormone is one of the most powerful levers on metabolism, which makes it tempting for people trying to get very lean. It is also a hormone with effects on almost every organ. This guide explains how T3 works, what doctors use it for and why its use for fat loss is risky.
T3, T4 and how the thyroid works
Your thyroid makes mostly T4 (thyroxine), which the body converts into T3, the active form. The pituitary controls the thyroid through TSH (thyroid-stimulating hormone). When thyroid hormone is high, TSH falls and the thyroid slows down. Levothyroxine (Synthroid, Eltroxin) is synthetic T4; liothyronine (Cytomel) is synthetic T3, which acts faster and wears off faster.
Medical use
Doctors prescribe thyroid hormone for hypothyroidism (an underactive thyroid). Most patients take levothyroxine; liothyronine is used in some cases, and in certain specialist situations such as thyroid cancer management.
Why it is used for fat loss
T3 increases the rate at which cells burn energy. During a long diet, the body lowers its own T3 to save energy, which is part of why fat loss slows (see metabolic adaptation). Taking T3 overrides that, so more calories are burned at rest.
The costs
Muscle loss
Excess thyroid hormone speeds up protein breakdown as well as fat burning. In people with an overactive thyroid, muscle wasting and weakness are classic symptoms. This is why T3 is usually discussed only alongside anabolic steroids in contest preparation, and why it is a poor choice for anyone trying to keep muscle naturally.
Heart
Thyroid hormone raises heart rate and the force of each beat. Too much causes palpitations, a fast resting heart rate and, especially in older people, atrial fibrillation, an irregular rhythm that raises stroke risk.
Bones
Long-term excess thyroid hormone speeds up bone loss.
Suppression
External T3 lowers TSH, so your own thyroid makes less. After stopping, there can be a period of low thyroid function, with fatigue, cold intolerance and weight regain, while it recovers.
Other effects
- Anxiety, irritability and poor sleep
- Heat intolerance and sweating
- Tremor
- Increased hunger
- Loose stools
Thyroid hormone and other drugs
T3 is often combined with anabolic steroids, clenbuterol, yohimbine or caffeine in contest preparation. Each adds to heart rate and the risk of rhythm problems. Thyroid hormone also changes how fast the body processes some medications, including blood thinners such as warfarin and some diabetes drugs. Anyone on regular medication should speak to a pharmacist before using it.
Monitoring
- TSH, free T3 and free T4 before, during and after use
- Resting heart rate each morning
- Home blood pressure
- An ECG if you are over 35 or have heart risk factors
Your doctor can order thyroid tests, or you can pay privately at LifeLabs or Dynacare. See our bloodwork guide.
If you think your thyroid is low
Fatigue, feeling cold, constipation, dry skin and weight gain can be signs of a genuinely underactive thyroid, which is common, especially in women. The right step is testing through your doctor rather than self-treating. Hypothyroidism is easily diagnosed with a TSH test and treated safely. See thyroid conditions and weight.
Safer ways to restart stalled fat loss
- Take a one to two week diet break at maintenance calories.
- Increase daily steps rather than adding more cardio sessions.
- Keep protein high and keep lifting heavy.
- Re-check your intake; small portions creep up over time.
See cutting without losing muscle.
Products and legal status
We carry Zionova Cytomel, with batch lab results on the product page. Liothyronine is a prescription drug in Canada.
Frequently asked questions
Does T3 burn muscle?
Excess T3 increases protein breakdown along with fat burning, so muscle loss is a real risk, especially without anabolic support.
Does T3 shut down your thyroid?
It lowers TSH, so your thyroid makes less hormone during use. Recovery after stopping can take weeks or longer.
What is the medical dose of Cytomel?
For mild hypothyroidism the Canadian label starts at 25 mcg a day, with maintenance usually 25–75 mcg a day, guided by blood tests.
Photo: Direct Media, StockSnap (public domain).
General information, not medical advice. Liothyronine is a prescription drug in Canada.
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