Liothyronine (T3)

What liothyronine is

Liothyronine is the synthetic form of triiodothyronine, T3 for short, and is prescription-only. In Germany it is approved as Thybon: to bridge phases of hypothyroidism around radioiodine treatment for thyroid carcinoma, for hypothyroidism with proven or suspected impaired conversion of T4 to T3, usually together with levothyroxine, and for the thyroid suppression test.

What is rated is the state of knowledge. As a hormone, T3 has been described for decades, and its side effects are documented. For the use at issue here, weight loss with a healthy thyroid, the study situation is thin and speaks against the advertised benefit.

How it works

The thyroid mainly produces levothyroxine (T4), a prohormone. The actual active principle is T3, most of which is formed from T4 in the liver, kidney and other organs. T3 acts via gene expression and increases oxygen consumption as well as sugar, fat and protein metabolism; at the same time it acts on the heart, circulation, bones and brain. According to the summary of product characteristics, the half-life is about one day.

When T3 is supplied from outside, the pituitary’s TSH falls and the body’s own thyroid is slowed down. The idea behind the misuse is a higher basal metabolic rate. The catch lies in the same mechanism: the increase affects not only fat tissue but also the heart, circulation and bones.

What the studies show

Kaptein 2009: systematic review

Kaptein 2009 (J Clin Endocrinol Metab) analyzed randomized and prospective studies on T3 and T4 in overweight and in diseases outside the thyroid. Under calorie restriction, T3 lowered TSH and T4; a consistent effect on weight loss, protein breakdown, basal metabolic rate or heart rate could not be established. The studies were few, small and often of low quality; a firm conclusion would require larger randomized trials.

Celi 2011: T3 versus T4 in hypothyroidism

Celi 2011 (J Clin Endocrinol Metab) treated 14 people with hypothyroidism in a double-blind crossover design with T3 or T4, each to a TSH of 0.5 to 1.5 mU/l. Under T3, weight was 68.5 versus 70.6 kg (p = 0.009), total cholesterol fell by 10.9 percent and LDL by 13.3 percent. Heart rate, blood pressure, exercise capacity and insulin sensitivity did not differ.

Sawin 1994: low TSH and atrial fibrillation

Sawin 1994 (NEJM) followed 2,007 people aged 60 and over without atrial fibrillation for 10 years. With a TSH of 0.1 mU/l or less, the cumulative rate of atrial fibrillation was 28 percent, with normal TSH 11 percent; after adjustment, the relative risk was 3.1 (1.7 to 5.5). The study did not distinguish between the body’s own hyperthyroidism and hormone intake, and concerned older people.

Skrzypiec-Spring 2025: poisonings

A systematic review found 34 published cases of poisoning with thyroid hormones over 30 years, all after misuse or doses outside the approval, with a rising trend especially since 2015. The most common cause was use for doping purposes. Two cases ended fatally through acute heart failure, both after an acutely high dose.

Handelsman 2023: elite sport

In 498 serum samples from Australian doping controls, 2 showed biochemical hyperthyroidism, 4 per 1,000 athletes. Of 509 doping control forms, 2 declared T4 and none T3. In controlled elite sport, there is thus little evidence of misuse.

Where the data stop

• Fat loss with a healthy thyroid. The available studies are small, old and inconsistent; an effect has been neither shown nor reliably ruled out.
• What happens to muscle mass. Whether more protein is broken down alongside fat under T3 could not be consistently clarified in the review.
• How common the misuse is. Population studies on use for weight loss are lacking; the poisoning review is based on published individual cases and cannot reflect the actual situation.
• Long-term consequences in young healthy people. The data on heart and bones come from thyroid therapy and from older people with low TSH, not from studies in young healthy people taking T3 to lose weight.

Status, approval and legal

Liothyronine is approved in Germany as Thybon 20 Henning and Thybon 100 Henning and is prescription-only. The indications are narrow: bridging around radioiodine treatment for thyroid carcinoma, hypothyroidism with impaired conversion, suppression test. Weight reduction is explicitly not among them.

In the US, liothyronine is approved as Cytomel; the prescribing information carries the highlighted warning “not for the treatment of obesity or for weight loss”.

In sport, thyroid hormones are not prohibited under the WADA Code; nor are they listed in the annex of the German Anti-Doping Act.

Safety

The summary of product characteristics lists palpitations as a very common and racing heart as a common side effect, as well as insomnia, nervousness, arrhythmias and angina pectoris. Overdose leads to a moderate to severe increase in metabolism; in older people and people with heart disease, a threat to breathing and circulation must be expected. Several cases of sudden cardiac death have been described for long-term thyroid hormone misuse. According to the summary of product characteristics, higher doses are particularly dangerous in combination with weight-loss agents.

The US prescribing information additionally warns that overdose increases bone loss and lowers bone density, and that in older people and people with heart disease the risk of atrial fibrillation rises.

If you notice racing heart, chest pain or severe restlessness while taking T3, have it checked by a doctor.

Sources

• Kaptein EM et al., J Clin Endocrinol Metab 2009 – thyroid hormone in obesity and other diseases, systematic review
• Celi FS et al., J Clin Endocrinol Metab 2011 – liothyronine versus levothyroxine in hypothyroidism, crossover RCT with 14 people
• Skrzypiec-Spring M et al., Pharmaceuticals 2025 – poisonings from misused thyroid hormones, systematic review
• Handelsman DJ et al., J Endocr Soc 2023 – thyroid hormone misuse among elite athletes, not banned by WADA
• Sawin CT et al., N Engl J Med 1994 – low TSH and atrial fibrillation in people over 60
• Summary of product characteristics Thybon 20/100 Henning (as of November 2024)
• FDA prescribing information Cytomel (DailyMed) – warning: not for weight loss