T3 vs. Semaglutide: What to Choose and for Whom

«I have a slow metabolism — maybe I need T3 rather than semaglutide?» The editors receive such a question regularly. The answer depends on whether the person has a genuine thyroid function disorder, obesity or diabetes. In this article we examine typical situations and explain how physicians determine who needs a thyroid hormone and who needs a weight-control drug.

Start with the Diagnosis, Not with the Drug

The feeling of a «slow metabolism» — fatigue, sensitivity to cold, weight gain, difficulty losing weight — can have very different causes. Among them are hypothyroidism, lack of sleep, a sedentary lifestyle, depressive states, side effects of medications, and insulin resistance. Therefore the first step should be not the choice of a drug, but finding out the cause.

To assess thyroid function, thyroid-stimulating hormone (TSH) and, if needed, free thyroxine (free T4) are usually measured. It is TSH that is the most sensitive indicator: in primary hypothyroidism it is elevated. Assessing the «state of the thyroid» on one's own solely by well-being or by the T3 level is incorrect.

To assess obesity and metabolic risk, the physician measures body weight, height and waist circumference, calculates the body mass index, checks blood pressure, fasting glucose or glycated hemoglobin, the lipid profile, and liver function.

Only after this does it become clear which category a person belongs to: hypothyroidism, obesity, diabetes, their combination, or none of these conditions. On this depends whether T3, semaglutide or neither is needed.

Who Needs Thyroid Hormones

Thyroid hormones are prescribed to people with confirmed hypothyroidism — a condition in which the gland produces insufficient hormones. The goal of treatment is to return TSH and hormone levels to normal, not to accelerate metabolism beyond the physiological level.

According to the guidelines of the American Thyroid Association (Jonklaas et al., 2014), the standard of treatment for hypothyroidism is levothyroxine (T4). The body itself converts it into active T3 in tissues, which provides a more stable hormone level. Liothyronine (T3) as monotherapy for long-term treatment is usually not recommended.

A combination of T4 and T3 is sometimes considered in patients who, despite a normal TSH on levothyroxine, continue to have symptoms. The European Thyroid Association in its 2012 guidelines allowed such a combination as an experimental approach for individual patients under the supervision of an endocrinologist.

For people without hypothyroidism, thyroid hormones are not indicated. The prescribing information for the drugs directly warns that they should not be used for weight loss, since large doses can cause severe, life-threatening toxicity.

Who Semaglutide May Be Indicated For

Semaglutide at the dosage for treating obesity (Wegovy), according to the prescribing information, is prescribed to adults with a body mass index of 30 kg/m² and above, or 27 kg/m² and above in the presence of at least one weight-related comorbidity, as an adjunct to diet and physical activity. For some patients with cardiovascular disease it also reduces the risk of cardiovascular events, as shown by the SELECT study.

Semaglutide drugs for treating type 2 diabetes (Ozempic, the tablet Rybelsus) are prescribed to improve glycemic control, and in people with high cardiovascular risk — also to reduce the risk of cardiac events.

Semaglutide is contraindicated in medullary thyroid cancer in one's personal or family history, in MEN 2 syndrome, and during pregnancy. Caution is needed for people with a history of pancreatitis, severe gastrointestinal diseases and diabetic retinopathy.

• The drug is effective only with long-term use: after discontinuation weight often returns.
• Part of the lost mass may come from muscle, so protein and strength training are important.
• The dose is escalated gradually under a physician's supervision to reduce gastrointestinal side effects.

Hypothyroidism and Obesity at the Same Time

A fairly common situation is a person having both hypothyroidism and obesity. In that case thyroid function is compensated first. Uncompensated hypothyroidism can contribute to weight gain through fluid retention and reduced energy expenditure, but after the hormones are normalized this part of the weight is usually small.

If after reaching a normal TSH the obesity persists, it is treated as an independent disease: change of nutrition, physical activity and, when indicated, pharmacotherapy, in particular GLP-1 agonists. The presence of compensated hypothyroidism by itself does not prevent the prescription of semaglutide.

Increasing the dose of thyroid hormones beyond what is needed in order to «help lose weight» is not allowed. The systematic review by Kaptein, Beale and Chan (2009) showed that such an approach is accompanied by loss of fat-free mass and adverse effects, while drug-induced thyrotoxicosis increases the risk of atrial fibrillation and loss of bone tissue.

Note: semaglutide slows gastric emptying, which theoretically may affect the absorption of oral drugs. People who take levothyroxine should inform the endocrinologist about starting GLP-1 therapy, so that TSH can be checked if needed.

People of Normal Weight and Athletes

For people of normal weight who want to «get lean», neither of the drugs is indicated. T3 in such a situation accelerates not only the breakdown of fat, but also the destruction of muscle proteins, which for an athlete means the loss of results gained with such difficulty, while the risks for the heart increase.

Semaglutide has not been studied as a means of reducing body fat percentage in people of normal weight. Appetite reduction against a background of intense training may make recovery and meeting protein needs more difficult.

Neither thyroid hormones nor GLP-1 agonists, at the time of writing, are included in the WADA Prohibited List. However, absence from the list does not mean safety: for athletes the health risks are an independent reason to refrain from non-medical use.

A more practical path is a moderate calorie deficit, adequate protein, strength training, full sleep and, if desired, supplements with proven safety under the control of a sports nutrition specialist.

Editorial Conclusions

The choice between T3 and semaglutide is primarily a question of diagnosis. Thyroid hormones are prescribed in confirmed hypothyroidism, and the standard is levothyroxine, not T3. Semaglutide is prescribed in obesity with indications and in type 2 diabetes.

If both conditions are present, thyroid function is compensated first, and then, if needed, obesity is treated. The use of T3 for weight loss is dangerous and not directly recommended in the prescribing information.

People of normal weight and athletes do not need either of these drugs.

We also recommend reading our materials on the TSH test and how to interpret it, on preserving muscle on GLP-1 agonists, and on myths about a «slow metabolism».

References

1. Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association task force on thyroid hormone replacement. Thyroid. 2014;24(12):1670–1751.
2. Wiersinga WM, Duntas L, Fadeyev V, et al. 2012 ETA guidelines: the use of L-T4 + L-T3 in the treatment of hypothyroidism. Eur Thyroid J. 2012;1(2):55–71.
3. Kaptein EM, Beale E, Chan LS. Thyroid hormone therapy for obesity and nonthyroidal illnesses: a systematic review. J Clin Endocrinol Metab. 2009;94(10):3663–3675.
4. Wilding JPH, Batterham RL, Calanna S, et al. Once-weekly semaglutide in adults with overweight or obesity. N Engl J Med. 2021;384(11):989–1002.
5. Lincoff AM, Brown-Frandsen K, Colhoun HM, et al. Semaglutide and cardiovascular outcomes in obesity without diabetes. N Engl J Med. 2023;389(24):2221–2232.
6. U.S. Food and Drug Administration. Wegovy (semaglutide) injection: prescribing information. Novo Nordisk.
7. U.S. Food and Drug Administration. Cytomel (liothyronine sodium) tablets: prescribing information.