Definiton
T4 is the short name of the iodine-containing thyroid hormone tetraiodothyronine. A common name is also thyroxine. 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.
Too low values indicate an underactive thyroid gland and too high values indicate an overactive thyroid gland. In the case of hypofunction, for example due to an inflammatory disease (frequently: Hashimoto’s thyroidoditits), the value is usually too low. In the case of hyperfunction, as is possible in Graves’ disease or thyroid autonomy, elevated 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 expressed in the unit nanogram per liter (ng/L) or per deciliter (ng/dl). One nanogram is one billionth of a gram (or 0.000000001 gram). Sometimes, however, the value is also given in the unit picomol per liter (pmol/L).
Instead of the weight, the number of particles is given. Since the thyroid hormone is predominantly bound to proteins and only a small part is “free” and thus directly available to the body, usually only the free T4 (fT4) is determined. The normal range of free T4 (fT4) concentration in the blood is 8-18 ng/liter or 10-23 pmol/liter.
If given per deciliter (tenth liter), the values are 0.8 to 1.8 ng/dl or 1 to 2.3 pmol/dl. The confusion is further increased by the fact that there are also other applicable limit ranges for different population groups such as pregnant women, the elderly or children. Whether the T4 value is within the normal range should therefore only be assessed by a doctor.
How does T4 change during pregnancy?
During pregnancy, hormonal changes occur, so that the T4 value often does not remain the same. A distinction is made between physiological changes, i.e. those that occur naturally during pregnancy, and pathological, i.e. pathological changes that require treatment.
The free T4 increases naturally in the first part of the pregnancy to a maximum value, which is usually reached between the ninth and 13th week of pregnancy. In the further course of the pregnancy, the value decreases again. However, the control hormone of the thyroid gland, thyrotropin or TSH, is usually first determined in the pregnant woman. The value of this hormone and the limits that apply depending on the phase of pregnancy can usually be used to determine whether the thyroid function is OK or whether there is under- or over-functioning.
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