What are thyroid levels?
The hormone production of the thyroid gland is adjusted to the respective demand in an interaction with the pituitary gland. The thyroid values in the blood therefore indicate not only how the thyroid gland itself is working, but also whether and how well the control loop is functioning.
A distinction is made between the TSH produced in the pituitary gland (“central thyroid level”) and the hormones T3 and T4 produced in the thyroid gland (“peripheral thyroid levels”).
TSH level
TSH (thyroid-stimulating hormone = thyrotropin) is secreted by the pituitary gland and reaches the thyroid gland with the blood. There it stimulates iodine uptake and the production of T4 and T3. If the concentration of these two thyroid hormones in the blood increases, the production of TSH decreases because the thyroid gland needs to be stimulated less. Thus, the pituitary gland and thyroid gland are interrelated.
Read more about this in the article TSH value!
T3 and T4
The biological half-life of T3 is about 19 hours: After this period, half of the original amount of hormone has been degraded. In contrast, T4 has a biological half-life of about 190 hours. In addition, about three times as much T4 as T3 circulates in the blood.
Effect of T3 and T4
Thyroid hormones control metabolism in many ways by influencing the synthesis of various proteins in the body’s cells. They also promote hormone secretion in certain organs, for example in the pancreas and adrenal gland. In childhood, thyroid hormones are of great importance for growth and brain development. In summary, the main effects of thyroid hormones are as follows:
- Increasing metabolic activity at rest (basal metabolic rate) and thus oxygen consumption.
- Promotion of protein synthesis, carbohydrate and fat metabolism
- @ Regulation of heat balance and body temperature
- Promoting growth, especially of the nervous and skeletal systems in embryonic and child development.
- increase of cholesterol excretion
When are thyroid levels determined?
Thyroid hormones are determined for the following issues:
- Is there hypothyroidism or hyperthyroidism?
- In the case of hyperthyroidism or hypothyroidism, is the hormonal control loop with the pituitary gland disturbed?
- Is there an underactivity of the pituitary gland?
- Is the thyroid gland inflamed?
- Is hypothyroidism being treated with the right amounts of hormones?
Furthermore, TSH is determined before every operation (anesthesia tolerance!) as well as before every radiological examination with iodine-containing contrast media. This value alone is usually sufficient here, because it also changes with thyroid function disorders.
Blood values: thyroid and pituitary gland
|
Thyroid value |
Normal value (blood serum) |
|
TSH-basal |
0.27 – 4.20 µIU/ml |
|
free T3 (fT3) |
2.5 – 4.4 ng/l (3.9-6.7 pmol/l) |
|
Total T3 |
0.8 – 1.8 µg/l (1.2-2.8 nmol/l) |
|
free T4 (fT4) |
9.9 – 16 ng/l (12.7-20.8 pmol/l) |
|
Total T4 |
56 – 123 µg/l (72-158 nmol/l) |
However, these reference ranges may differ from laboratory to laboratory as they use different measurement methods. In children, higher standard values apply depending on age; in the elderly, lower values tend to apply.
In practice, the physician does not always determine all thyroid values. For example, the TSH value is sufficient to exclude a primary thyroid disorder. In addition, the values of the free thyroid hormones are more informative than the total values, since only the former are biologically active. To determine hypothyroidism, the physician usually determines the levels of TSH and fT4. For the diagnosis of hyperthyroidism, TSH, fT4 and fT3 are important.
When are the thyroid values increased or decreased?
Sometimes, however, the pituitary gland also produces insufficient TSH (and other hormones). This is called pituitary insufficiency. Very rarely, a tumor in the pituitary gland can also produce too much TSH. If the TSH value is altered, T3 and T4 are also determined. This results in typical constellations of thyroid values in different diseases:
TSH increased, T3 and T4 decreased.
This constellation indicates an underactive thyroid (hypothyroidism). Values of T3 and T4 are decreased because the thyroid gland produces insufficient amounts of both hormones. In response, the pituitary gland attempts to increase thyroid function with increased secretion of TSH. Hypothyroidism occurs mainly in autoimmune thyroid diseases (such as Hashimoto’s thyroiditis).
TSH decreased, T3 and T4 increased
- Graves’ disease or thyroiditis in acute episode
- autonomous hormone-producing thyroid adenoma (“hot nodule” )
- thyroid enlargement (goiter, “goiter”)
TSH increased/decreased, T3 and T4 normal
In the case of incipient (latent) hypothyroidism or hyperthyroidism, the thyroid gland is also disturbed. However, the T3 and T4 values are (still) normal because the pituitary gland counteracts this by increasing or decreasing the TSH values.
TSH decreased, T3 and T4 decreased
This constellation of values indicates the rare hypofunction of the pituitary gland (more precisely: anterior pituitary insufficiency). It should actually produce more TSH if T3 and T4 are too low. However, this is not possible with pituitary gland hypofunction.
TSH normal/increased, T3 and T4 increased.
In case of pituitary hyperfunction, the opposite happens: the pituitary gland does not reduce TSH secretion when T3 and T4 levels increase. Sometimes it even produces more TSH (for example because of a tumor), then the TSH value in the blood is also increased. Like hypofunction of the pituitary gland, hyperfunction is also very rare.
Yet another condition can increase TSH, as well as T3 and T4 levels: Thyroid hormone resistance. In this very rare hereditary disease, the gene of the T3 receptor is altered and defective.
It may also be that the level of only one of the hormones T3 or T4 is altered. In the early stages of hyperthyroidism, for example, T3 is elevated but T4 is not. In extreme iodine deficiency, T3 is elevated but T4 is decreased.
Altered thyroid values: What to do?
If one or more thyroid values are altered, an endocrinologist (specialist in hormone disorders) should initiate further investigations to determine the cause.
In most cases, a sonography (ultrasound examination) of the thyroid gland is performed first to examine its structure more closely. This can reveal changes in size and condition. In addition, the metabolic activity of the thyroid gland can be determined by a so-called scintigraphy. Sometimes the thyroid gland must also be punctured to obtain a tissue sample – for example, if cancer is suspected.
If the cause of the altered thyroid values has been found, in many cases a therapy with medication can be started.
