Thyroid hormone T4 - thyroxine
Definition
T4 is the abbreviation for the iodine-containing thyroid hormone tetraiodothyronine. Thyroxine is still a common name. T4 and the structurally related T3 (triiodothyronine) are involved in numerous metabolic processes in the body and are essential for the proper functioning of the body. Values that are too low indicate an underactive thyroid and values that are too high indicate an overactive thyroid.In the case of hypofunction, for example due to an inflammatory disease (often: Hashimoto's thyroiditis), the value is usually too low. In cases of hyperfunction, as is possible with Graves' disease or thyroid autonomy, increased T4 values are often measured. If T4 is not in the normal range, the doctor usually has to order treatment with tablets and, if necessary, further diagnostics.
What are the normal values for T4?
The thyroid hormone T4 can be determined in the blood and its concentration is usually given in the unit nanograms per liter (ng / L) or per deciliter (ng / dl). A nanogram is a billionth of a gram (or 0.000000001 gram). In some cases, however, the value is also given in the unit picomole per liter (pmol / L). Instead of weight, it indicates the number of particles. Since the thyroid hormone is mainly bound to proteins and only a small part is "free" and thus directly available to the body, only the free T4 (fT4) is usually determined. The normal range for the concentration of free T4 (fT4) in the blood is 8-18 ng / liter or 10-23 pmol / liter.
If the information is given per deciliter (tenths of a liter), the values are 0.8 to 1.8 ng / dl or 1 to 2.3 pmol / dl. The confusion is increased by the fact that there are also other applicable border areas for different population groups such as pregnant women, the elderly or children. Whether the T4 value is in the normal range should therefore only be assessed by a doctor.
How does T4 change during pregnancy?
During pregnancy there are hormonal changes, so that the T4 value often does not stay the same. A distinction is made here between physiological changes, i.e. those that occur naturally during pregnancy, and pathological changes, i.e. pathological changes that require treatment. The free T4 naturally increases in the first part of pregnancy up to a maximum value, which is usually reached between the ninth and 13th week of pregnancy. In the further course the value decreases again.
However, the control hormone of the thyroid gland, thyrotropin or TSH, is usually determined in the pregnant woman. Using its value and the limit values that apply depending on the phase of pregnancy, it can usually be determined whether the thyroid function is okay or whether there is an underactive or overactive condition.
T4 value and desire for children
A normal thyroid function in women is very important if they wish to have children. Therefore, the value for free T4 and the control hormone TSH should be in the normal range. Both underactive and overactive, or too low and too high T4 values can lead to severe permanent damage in the development of the child. In addition, the underactive function is a possible reason why the woman cannot become pregnant in the first place. However, if pregnancy occurs despite a thyroid dysfunction, the risk of miscarriage is increased. An increase or decrease in the T4 value should therefore be clarified and, if necessary, treated if you want to have children. In addition, if the onset of pregnancy is successful, the recommended iodine precaution should be adhered to. The T4 value and thus the thyroid function in men has no direct influence on the desire to have children.
Why is my T4 value too high?
If the T4 value is too high, this indicates an overactive thyroid. This, in turn, can have various causes, some of which are quite common and others rather seldom.
One of the possible more common causes is what is known as thyroid autonomy. This leads to uninhibited, excessive hormone production by individual areas of the thyroid gland. The diagnosis is made using thyroid scintigraphy. For this purpose, a small amount of radioactive iodine is added to the blood, which accumulates in the thyroid gland. In the case of thyroid autonomy, an area of excessive accumulation is shown in the graphic, which is then referred to as a “hot lump”.
The second common cause of a too high T4 value is Graves disease. The excessive hormone production is triggered by so-called autoantibodies, which are directed against thyroid receptors. In this way, the thyroid gland is driven to produce excessive hormones. The diagnosis of Graves' disease is made possible primarily by determining these autoantibodies in the blood.
Another possible cause of elevated T4 levels is excessive use of thyroid medication if, for example, the dose set by the doctor is too high. Depending on the underlying cause of the increased T4, targeted therapy can be initiated.
Why is my T4 value too low?
A too low T4 value indicates a thyroid hormone deficiency, which is usually due to an underactive thyroid. The subfunction can have various causes.
The thyroid disease Hashimoto's thyroiditis is quite common in the population (especially women). In this disease, the body produces special proteins (autoantibodies) that lead to inflammation and impaired function of the thyroid tissue.
Low T4 values also occur if the thyroid gland has been operated on or if radiation has been given to radioactive iodine due to illness. After such an operation, patients usually have to take thyroid hormones as tablets for their entire life. If the dosage is too low, there will be hypofunction and the T4 value is too low.
T4 as a drug
T4 is not only an important hormone produced by the body but also a very often prescribed drug. People with an underactive thyroid (for example, as a result of a thyroid operation or an inflammation of the thyroid gland) are dependent on taking T4 in order to lead a normal life. The tablet, which is usually taken once in the morning, replaces the function that the thyroid gland otherwise takes on, and the daily requirement for thyroid hormone is covered. An individual dose has to be set for each person for their respective needs. By regularly checking the blood values, the doctor can check this dose and adjust it if necessary.
Medicines with the active ingredient T4 (tetraiodothyronine, often also referred to as thyroxine) are available from different manufacturers in different dosages. The trade name differs depending on the manufacturer. However, the active ingredient and effect are identical. Frequently prescribed preparations are, for example, L-Thyroxin Henning or Euthyrox. The usual dosages are in the range of 25 to 300 µg (micrograms). There are also drugs that contain iodine in addition to T4, such as thyroid iodine or iodothyrox.
T4 for weight loss
If you want to lose weight, taking T4 is by no means an option. However, some people cause weight loss by taking thyroid hormones. On the one hand, this is usually unsuccessful and, on the other hand, it is very dangerous. The thyroid hormones (such as T4) lead to an increase in the body's energy expenditure, so that people with an overactive thyroid often lose weight.
However, if you take thyroid hormones like T4 without relying on them for a thyroid disorder, it can lead to artificially induced hyperthyroidism. In addition to complaints such as restlessness, tremors, sweating or diarrhea, life-threatening consequences such as cardiac arrhythmias can occur. Those who would like to lose weight and also want to maintain their weight in the long term can only achieve this by changing their eating habits (lower calorie intake) and through increased physical activity (higher calorie consumption).
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T3 vs T4 - What's the Difference?
Both T4 and T3 are iodine-containing hormones that are produced by the thyroid gland. They only differ chemically in that T3 (triiodothyronine) contains three iodine particles and T4 (tetraiodothyronine) four. While T4 is more stable and disintegrates less quickly, T3 is up to a hundred times more effective than T4. The body is also able to convert T4 to T3.
The thyroid mainly produces T4, which can also be viewed as a kind of “storage form” and is constantly available in the blood. If necessary, the organism can fall back on this “supply” and convert T4 into T3. For this reason, thyroid hormone replacement tablets usually contain the more stable T4. In the blood test for thyroid values, both T3 and T4 are usually determined. With a slight hormone deficiency, often only the T4 value is initially lowered. If there is a pronounced deficiency, however, both values are usually too low.
