What is SMP blood test
What are thyroid levels?
The hormone production of the thyroid gland is adapted to the respective need in an interrelationship with the pituitary gland. The thyroid values in the blood therefore not only show how the thyroid is working, but also whether and how well the control loop is working.
A distinction is made between the TSH produced in the pituitary gland (“central thyroid value”, thyrotropin) and the hormones T3 and T4 (“peripheral thyroid value”) produced in the thyroid.
TSH (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 less stimulation. The pituitary gland and thyroid are thus interrelated.
You can read more about this in the article TSH value!
T3 and T4
T3 (triiodothyronine) and T4 (thyroxine, L-thyroxine) are mostly bound to so-called transport proteins in the blood. However, only unbound, ie “free” T3 and T4 (precise designation: fT4 and fT3) can act as a messenger substance in the body. Here, fT3 is the more effective of the two thyroid hormones. T4 serves primarily as a precursor (prohormone) for T3. It is converted into T3 in the cells of the body as required.
The biological half-life of T3 is about 19 hours: after this period of time, half of the original amount of hormones is broken down. In contrast, T4 has a biological half-life of around 190 hours. In addition, about three times as much T4 as T3 circulates in the blood.
Effect of T3 and T4
Thyroid hormones control the metabolism in many ways by influencing the synthesis of various protein substances in the body's cells. They also promote hormone release in certain organs, for example in the pancreas and adrenal glands. In childhood, the thyroid hormones are of great importance for growth and brain development. In summary, the main effects of thyroid hormones are:
- Increase in metabolic activity at rest (basal metabolic rate) and thus oxygen consumption
- Promotion of protein synthesis, carbohydrate and fat metabolism
- Regulation of the heat balance and body temperature
- Enhancement of the effects of the stress hormones noradrenaline and adrenaline (heart rate and blood pressure rise and the heart beats faster)
- Promotion of growth, especially of the nervous and skeletal system in embryonic and child development
- Increase in cholesterol excretion
When are the thyroid levels determined?
The thyroid hormones are determined for the following questions:
- Is there an underactive or overactive thyroid?
- Is the hormonal control loop with the pituitary gland disturbed in the event of over- or under-functioning?
- Is there an underactive pituitary gland?
- Is the thyroid inflamed?
- Is an underactive thyroid being treated with the right amounts of hormones?
Furthermore, the TSH is determined before each operation (tolerance to anesthesia!) And before each radiological examination with iodine-containing contrast media. This value alone is usually sufficient here because it also changes in the event of thyroid dysfunction.
Blood values: thyroid and pituitary gland
The following normal thyroid values apply to adults:
Other normal values apply to children depending on their age.
When are the thyroid levels increased or decreased?
Normal thyroid values indicate a normally functioning thyroid and an intact control loop. Changed values are mainly present when the thyroid gland produces too little or too much thyroid hormones.
Sometimes the pituitary gland also produces insufficient thyrotropin (and other hormones). One then speaks of pituitary gland insufficiency. Very rarely, a pituitary gland tumor can also produce too much thyrotropin. If the TSH value has changed, T3 and T4 are also determined. This results in typical constellations of thyroid values for various diseases:
Thyrotropin increased, T3 and T4 decreased:
This constellation speaks for an underactive thyroid (hypothyroidism). Values of T3 and T4 are lowered because the thyroid gland does not produce enough of the two hormones. In response to this, the pituitary gland tries to increase thyroid function with an increased release of thyrotropin. Hypothyroidism mainly occurs in autoimmune thyroid disorders (such as Hashimoto's thyroiditis).
Thyrotropin decreased, T3 and T4 increased:
With an overactive thyroid (hyperthyroidism), the peripheral thyroid values are too high. Therefore, the pituitary gland tries to counteract this by reducing TSH production. Elevated T3 & T4 values occur in the following diseases:
- Graves' disease or thyroid inflammation in the acute episode
- autonomous hormone-producing thyroid adenoma ("hot knot")
- Thyroid enlargement (goiter, "goiter")
Thyrotropin decreased, T3 and T4 decreased:
This constellation of values indicates the rare underactive pituitary gland. You should actually make more thyrotropin if T3 and T4 are too low. However, this is not possible if the pituitary gland is underactive.
Danger: Changes in thyroid values are not always caused by a disease, but sometimes also by certain medications. For example, the release of TSH can be reduced by the anticoagulant heparin and increased by certain active ingredients against psychiatric diseases (such as haloperidol).
Changes in thyroid levels: what to do?
If one or more thyroid values are changed, an endocrinologist (specialist in hormone disorders) should initiate further examinations to determine the cause.
Usually a sonography (ultrasound examination) of the thyroid gland is carried out first in order to examine its structure more closely. Changes in size and texture can be revealed. A so-called scintigraphy can also be used to determine the metabolic activity of the thyroid. Sometimes the thyroid gland also has to be punctured to obtain a tissue sample - for example if cancer is suspected.
Became the cause of the changed Thyroid levels found, therapy with medication can be started in many cases.
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