The thyroid hormone T4 (Thyroxine) is a major hormone secreted by the thyroid gland that plays a key role in regulating metabolism, body temperature, heart rate and a variety of body functions. T4 is converted into the more active form T3 (triiodothyronine) in the body, which together regulate metabolic rate and energy expenditure. Based on T4's role in regulating metabolism, bodybuilders use it to help build muscle and reduce fat. 

What is the difference between T4 and T3?

T4 (thyroxine, tetraiodothyronine) and T3 (triiodothyronine) are two hormones secreted by the thyroid gland that play a key role in regulating the body's metabolic function and energy expenditure. Although they both belong to the thyroid hormones, there are some important differences between them:

The main difference between T4 and T3

T4 vs T3 Chemical structure:

T4: Full name of tetraiodothyronine, the molecule contains four iodine atoms.

T3: Full name of triiodothyronine, the molecule contains three iodine atoms.

T4 vs T3 activity Strength:

T3 is the more active form: T3 is about 3-5 times more biologically active than T4. This means that T3 binds to cell receptors more efficiently, initiating metabolism-related responses.

T4 is the storage form: T4 is a precursor hormone whose main role is to provide the body with the raw material for T3. Most T4 is converted into T3 in the body, where it plays a physiological role.

The ratio of T4 vs T3 in the blood:

The concentration of T4 is higher: under normal conditions, the blood concentration of T4 is much higher than that of T3, about 20:1 ratio. This is because T4 is primarily a storage form, and the body converts T4 into T3 as needed.

Low concentration of T3: Despite its low concentration, T3 is the most active thyroid hormone and is directly involved in the regulation of metabolism.

T4 vs T3 half-life:

T4 has a long half-life: The half-life of T4 is about 7 days, so the concentration in the blood is relatively stable.

The half-life of T3 is shorter: the half-life of T3 is about 1 day, and the action is shorter, but the reaction is faster.

T4 vs T3 function:

T4: acts primarily as a reserve hormone and is converted to T3 when needed. Therefore, the direct metabolic effect of it itself is weak.

T3: It is the main active hormone that regulates the body's metabolism, heart rate, body temperature, energy expenditure, etc.

T4 To T3 conversion process:

T4 conversion to T3: Most of the T3 in the body is converted from T4, and this conversion takes place mainly in the liver and kidneys. deiodinase is the enzyme responsible for this process, producing T3 by removing an iodine atom from the T4 molecule.

T4 vs T3 metabolic regulation:

The action of T3 is more direct and rapid: T3 directly affects cellular metabolic rate, regulating protein synthesis, lipolysis, and carbohydrate utilization.

The action of T4 is more indirect: T4 regulates metabolism by being converted into T3, but its own action is slower.

Differences in medical application of T4 vs T3

To treat hypothyroidism:

T4 (Levothyroxine) is a common drug used to treat hypothyroidism (low thyroid function). Because T4 can be converted into T3 in the body, it is able to provide a continuous supply of hormones.

T3 (Liothyronine) is also sometimes used therapeutically, but because of its strong activity and short half-life, it is usually used only in acute treatment or in specific situations, such as thyroid crises.

T4 vs T3 Metabolic disease Treatment:

T4 is often used for long-term maintenance because it is relatively smooth and provides the body with the necessary thyroid hormone.

T3 is often used as a short-term treatment, especially if a rapid metabolic response is required.

What does low T4 mean?

Low levels of T4 (thyroxine) usually indicate hypothyroidism, also known as hypothyroidism. Hypothyroidism is a common endocrine disorder in which the thyroid gland does not produce enough thyroid hormone to maintain normal metabolic function. T4 is one of the main hormones secreted by the thyroid gland, and when its level is too low, many physiological processes in the body can slow down, leading to various health problems.

Possible causes of low T4 levels

1. The most common cause is primary hypothyroidism, which includes autoimmune diseases (such as Hashimoto's thyroiditis), iodine deficiency, thyroid surgery, or radiation therapy.

2. Secondary hypothyroidism, such as pituitary insufficiency, may result in insufficient T4 secretion.

3. Drug effects: Certain drugs, such as lithium (used to treat bipolar disorder) or anti-thyroid drugs, inhibit thyroid hormone synthesis. Adrenal steroids or certain chemotherapy drugs may also interfere with thyroid hormone production.

4. Thyroiditis: Subacute thyroiditis or other types of inflammation of the thyroid may disrupt the normal function of the thyroid gland, resulting in reduced thyroid hormone production.

Low T4 levels are often accompanied by fatigue and fatigue, weight gain, cold intolerance, depression and low mood, dry skin, thinning hair, constipation, a slow heart rate and other symptoms that reflect a slowing down of metabolism in the body.

T4 levels can be detected with a blood test, usually along with thyroid stimulating hormone (TSH) levels. If T4 is low and TSH is also low, it may be secondary hypothyroidism, meaning the problem lies with the pituitary or hypothalamus rather than the thyroid itself.

Conversely, a high level of T4 may indicate hyperthyroidism (hyperthyroidism), in which the thyroid gland is overactive and produces too many hormones. Symptoms include increased heart rate,  weight lose, anxiety, sweating and more.

Is there a relationship between T4 and weight lose?

There is a direct relationship between T4 (thyroxine) and weight lose, as T4 is one of the important hormones that regulate the body's metabolic rate. Thyroid hormones (including T4 and T3) play a key role in controlling the body's metabolism, energy expenditure, lipolysis, and protein synthesis. Therefore, changes in T4 levels can affect weight management and lose weight.

T4 and weight lose

Regulation of metabolism:

T4 is the main hormone secreted by the thyroid, and although it is the less active form, it can be converted into the more active T3 in the body, which affects the metabolic activity of cells. When T4 levels are normal, the body's metabolic rate is also maintained in the normal range, which helps balance weight and energy expenditure.

Low T4 levels (hypothyroidism) and weight gain:

When T4 levels are low, thyroid hormone production is insufficient, causing the metabolism to slow down. This means the body burns fewer calories and stores more fat, which leads to weight gain. Many patients with hypothyroidism will report an inability to lose weight or unexplained weight gain, despite no significant changes in their diet and exercise habits.

People with hypothyroidism often have water retention and fat accumulation, which is also a cause of weight gain.

High T4 levels (hyperthyroidism) and weight loss:

When T4 levels are too high (as in hyperthyroidism or excessive use of thyroid hormone supplements), the body's metabolism speeds up and energy expenditure increases. This can lead to rapid weight loss, even if the amount of food eaten is not reduced. In this condition, the body burns fat and muscle tissue at an increased rate, leading to weight loss.

However, hyperthyroidism can also cause side effects such as muscle loss, increased heart rate, anxiety, and is not a healthy way to lose weight.

Effects of low T4 levels on weight loss

Difficulty in losing weight:

For patients with hypothyroidism, because the metabolism is slow and the body burns fewer calories, even if the diet is controlled and the amount of exercise is increased, the weight loss effect may not be ideal. This often leaves patients feeling frustrated.

Risk of weight gain:

People with low T4 levels are more prone to fat accumulation, especially in the abdomen, which increases the risk of obesity. This not only affects appearance, but may also increase the risk of chronic diseases such as cardiovascular disease and diabetes.

Treatment for hypothyroidism helps with weight loss:

For patients with hypothyroidism, drugs such as Levothyroxine are supplemented to restore normal T4 levels, which can often gradually restore metabolic function, help control weight and promote weight loss.

Some bodybuilders who want to lose weight quickly use T4 or T3 supplements to increase metabolism and speed up fat breakdown.