The thyroid hormone T3 (Triiodothyronine) is one of two main hormones secreted by the thyroid gland, the other being T4 (thyroxine). T3 is the most active form of thyroid hormone, which directly regulates the body's metabolism, body temperature, heart rate and other important physiological functions. Although the thyroid gland secretes much less T3 than T4, about 80% of T3 is converted through T4 in the body.

The relationship between T3 and T4:

T3 and T4 work together to maintain normal metabolism, with T4 being the storage form and T3 being the active form for immediate use. T4 is stable and slow to take effect, while T3 is fast and powerful.

Normal T3 levels:

Normal range of serum T3:80-180 ng/dL.

Free T3 (FT3, the part not bound to protein) Normal range: 2.3-4.1 pg/mL.

T3 levels in the blood are often tested to assess hyperthyroidism or hypothyroidism.

What is the difference between T3 and T4?

T3 is the more active thyroid hormone that acts directly on cells and regulates metabolism. T4 is a precursor hormone that is converted into T3 in the body before it can play its role. Therefore, the metabolic effects of T3 are stronger, but shorter in duration.

What does low T3 mean?

Low levels of T3 (triiodothyronine) usually indicate hypothyroidism, also known as hypothyroidism. T3 is one of the two main hormones produced by the thyroid gland (the other being T4), and it is an active form of metabolism that regulates the body's energy use, heart function, rate of digestion, and many other physiological functions. Low levels of T3 can negatively affect many systems in the body.

Primary hypothyroidism, thyroid switching dysfunction, secondary hypothyroidism, poor medication or nutrition, and insufficient thyroid-stimulating hormone (TSH) produced by the pituitary or hypothalamus can all lead to reduced production of T3 and T4.

Symptoms of low T3 levels often include fatigue and weakness, weight gain, depression and low mood, dry skin and hair loss, cold intolerance, slowed heart rate, constipation and other symptoms associated with slowed metabolism.

Detection and diagnosis of low T3 levels

Blood tests: When T3 levels are tested, T4 and TSH levels are usually also tested. Low T3 often occurs in conjunction with high TSH and low T4, especially in cases of hypothyroidism.

Detection of T3 and T4 conversion: If T4 levels are normal or high, but T3 levels are low, it may indicate that T4 is not effectively converted to T3.

Treatment of low T3 levels

Thyroid hormone replacement therapy:

Levothyroxine (T4) : This is the most common treatment for hypothyroidism, and when T4 is supplemented, the body converts part of it into T3, which increases T3 levels.

In some cases, doctors may use Liothyronine (T3) directly, especially if T4 supplementation is not working well or the body is unable to convert T4 to T3.

Improved conversion from T4 to T3:

By improving diet, managing stress, and treating liver or kidney problems, it may help to improve the efficiency of T4 to T3 conversion.

Certain nutrients, such as selenium and zinc, are very important for the conversion of T4 to T3, and deficiencies in these trace elements may affect T3 levels.

Treatment of underlying diseases:

If low T3 levels are due to a chronic disease or medication, prioritizing treatment or adjusting the underlying cause is key to restoring T3 levels.

Why supplement T3 and not just T4?

The reason to supplement T3 (triiodothyronine) rather than just T4 (thyroxine) is mainly related to the physiological role, activity and the body's need for them. Here are some of the main reasons to supplement T3:

T3 is the more active thyroid hormone

T3 is one of the main hormones secreted by the thyroid gland, and it is an active form of metabolism. Although T4 is more abundant in the body, T4 needs to be converted into T3 in cells to function.

T3 acts directly on cells, regulating metabolism, energy expenditure, heart function and growth and development. T3 supplementation is able to rapidly increase the level of active thyroid hormone in the body, which can quickly improve symptoms.

Have insufficient ability to transform T4

Some people (e.g., those with certain chronic conditions, liver or kidney dysfunction) may not be able to effectively convert T4 to T3. This means that even when T4 is supplemented, T3 levels in the body are still insufficient, leading to slower metabolism and associated symptoms.

For these people, T4 supplementation alone may not meet the body's needs, so direct T3 supplementation is needed.

Faster curative effect

T3 supplementation can quickly improve symptoms of hypothyroidism, such as fatigue, weight gain and low energy. In contrast, supplementing T4 can take several weeks for optimal results because the body needs time to convert T4 into T3.

Individualized therapy

Different individuals have different requirements for thyroid hormones. Some patients may experience discomfort when treated with T4, or their symptoms may not be adequately relieved. By combining T3 supplementation, the treatment regimen can be personalized to achieve better results.

Some doctors use a combination of T4 and T3 to ensure that their patients' hormone levels are at their best.

How does T3 triiodothyronine affect weight loss?

T3 (triiodothyronine) plays an important role in weight loss because it is a key thyroid hormone that regulates metabolism. T3 affects weight management by speeding up metabolic rate and promoting lipolysis and energy expenditure. Therefore, the level of T3 is closely related to weight gain or loss.

The mechanism by which T3 affects weight loss

Increase basal metabolic rate (BMR) :

T3 is the most active of the thyroid hormones and directly affects the body's basal metabolic rate (BMR), which is the amount of energy the body expends during resting. T3 can accelerate cell metabolic activity, increase the breakdown of fats, carbohydrates and proteins, and promote Calorie consumption.

At high T3 levels, the body burns more energy, and even if you don't increase your exercise or reduce your diet, your body's caloric expenditure will increase, which will help you lose weight.

Promotes lipolysis:

T3 helps break down fat stored in the body by promoting lipolysis (the process of lipolysis) in fat cells, converting it into energy. T3 activates certain enzymes involved in lipolysis, causing fat to be broken down more quickly.

This helps to lose fat and reduce fat stores in the body, especially stubborn subcutaneous fat and visceral fat.

Affects glucose and carbohydrate metabolism:

T3 also regulates carbohydrate metabolism and, by affecting insulin sensitivity, helps cells use glucose more efficiently, thereby reducing the chances of fat storage. At higher T3 levels, glucose metabolism is more efficient and the body is less likely to convert excess carbohydrates into fat storage.

Regulating protein metabolism:

T3 not only accelerates fat breakdown, but also increases protein metabolism. This means that T3 promotes protein synthesis and breakdown, which in turn affects muscle mass. If T3 levels are too high, it may trigger muscle breakdown, which is a side effect to be aware of when overusing T3 drugs. Therefore, it is important to maintain a proper balance of T3 levels when losing weight to avoid over-breaking down muscles.

Improve energy levels and endurance:

T3 promotes a faster metabolism and generally makes people feel more energetic and have more motivation to be physically active. This is very helpful for increasing exercise and increasing calorie expenditure, especially for those who feel sluggish due to a slow metabolism.

Relationship between T3 levels and weight loss

Low T3 levels make weight loss difficult:

If T3 levels are low (as in hypothyroidism), the metabolism slows down, causing the body to burn fewer calories. Even with a strict diet and increased exercise, weight loss can become difficult, and weight may rise instead of falling. Many patients with hypothyroidism experience weight gain and difficulty losing weight precisely because of insufficient T3 levels.

High T3 levels may promote rapid weight loss:

In the case of elevated T3 levels (such as hyperthyroidism or exogenous T3 supplementation), the metabolic rate is significantly elevated and the body burns fat and calories faster, leading to weight loss. When many people use T3 drugs, they will lose weight rapidly, and the speed of fat loss is also significantly faster.