The thyroid gland, which is located at the front of the neck just below the Krikoid, consists of 2 lobes, which are connected together by an isthmus. Follicular cells within the gland produce the 2 major thyroid hormones tetraiodothyronine (thyroxine, T4) triiodothyronine (T3) These hormones act almost all tissues of the body in cells, by binding to receptors of the cell nuclei and so influence the expression of a whole series of genes. Thyroid hormones are needed in the fetus and the newborn for normal development of the brain and body tissues and regulate in people of all ages the metabolism of proteins, carbohydrates and fats. T3 is the active form of the hormone that binds to the to the receptor in the cell nucleus, T4 has minimal hormonal activity. T4 is the much more stable form, can in most tissues are converted to T3 and thus serves as a reservoir for T3. A third form of thyroid hormone, reverse T3 (rT3), shows no metabolic activity. The levels of rT3 increase in certain (Editor’s note: severe) of diseases. Parafollicular cells (C-cells) secrete the hormone calcitonin, which will be distributed in the case of hypercalcemia and decrease serum calcium levels can (overview of disorders of the calcium concentration: regulation of calcium metabolism). Synthesis and release of thyroid hormones for the synthesis of thyroid hormones is iodine used (synthesis of thyroid hormones). Iodine, which is taken as iodide with the food is actively concentrated by the thyroid gland in the follicular cells and transferred by the thyroid peroxidase in organic iodine (organification). The follicular cells surround a space which is filled with colloid consisting of thyroglobulin, a glycoprotein which contains in its matrix tyrosine. Tyrosine is iodinated bodies in contact with the membrane of the follicular cells in a (monoiodotyrosine) or two (diiodotyrosine), and then each 2 molecules are joined together. Thus, the two thyroid hormones are formed (diiodotyrosine + diiodotyrosine ? T4; diiodotyrosine + monoiodotyrosine ? T3). Synthesis of thyroid hormones T3 and T4 remain stored within the follicle in thyroglobulin until the follicular cell thyroglobulin receives as colloid droplets. Inside the thyroid follicular cell are cleaved from thyroglobulin T3 and T4. Free T3 and T4 are then released into the bloodstream, where they are bound for the transport to serum proteins. The most important serum protein is thyroxine-binding globulin (TBG) which binds with high affinity equimolar T3 and T4. TBG normally binds 75% of the available to bind thyroid hormone. The other binding proteins, thyroxine binding prealbumin (transthyretin) which is a high affinity, but has a low capacity for T4, and albumin, which has a low affinity for, but a high capacity for T3 and T4. Approximately 0.3% of the total serum T3 and 0.03% of the total serum T4 are present as free hormone in equilibrium with bound hormone. Only free T3 and T4 can be effective in the periphery. All processes, which are necessary for the formation and release of T3 and T4 are of thyroid-stimulating hormone (TSH), which is secreted from the pituitary thyrotropic cells monitored. The TSH secretion is controlled by a negative feedback mechanism in the pituitary gland: Elevated levels of free T3 and T4 inhibit the synthesis and secretion of TSH while falling mirror increase the TSH secretion. The TSH secretion is also influenced by thyrotropin-releasing hormone (TRH), which is formed in the hypothalamus. The exact mechanisms that regulate the TRH synthesis and release are unclear. However, we know that a negative feedback by thyroid hormones prevents the TRH synthesis. The largest proportion of the circulating T3 is formed outside of the thyroid gland by Monodejodination from T4. Only a fifth of circulating T3 is secreted directly by the thyroid gland. Laboratory tests of thyroid function determination of TSH TSH value the most sensitive parameter for evaluation of thyroid function (see Table: Results of thyroid function tests in different clinical situations). Main results include hyperthyroidism or hypothyroidism with a high probability of, except in patients with central hypothyroidism due to disease in the hypothalamus or pituitary gland or very rarely in patients with pituitary resistance to thyroid hormone. Serum TSH may be false-low in critically ill patients. About the serum TSH levels reveals a subclinical hyperthyroidism (low serum TSH) and subclinical hypothyroidism (elevated serum TSH). In both cases, the values ??for serum T4, free T4, serum T3 and free T3 are normal. Results of thyroid function tests in different clinical situations physiological condition serum TSH free serum T3 T4 24-hour uptake of radioiodine hyperthyroidism Untreated Low * Large bulk T3 toxicosis low Normal United Normal or hypothyroidism primary, untreated bulk Low Low or normal or low-normal consequence of a low or normal pituitary Low Low or normal or low-normal euthyroid Patient takes iodine Normal Normal Normal low patient takes exogenous thyroid hormones Normal Normal patients with ingestion of T4, low in patients with ingestion of T3 high in patients with ingestion of T3, normal in patients with ingestion of T4 low patient takes estrogens a Normal normal large normal Low-T3 / low T4 syndrome ( “euthyroid sick syndrome”) normal, low or high normal or low low normal TSH is low in hyperthyroidism, except in the rare case when the hyperthyroidism by a TSH-secreting pituitary tumor or pituitary resistance to thyroid hormone is due. T3 = tri-iodothyronine; T4 = thyroxine, TSH = thyroid-stimulating hormone. T 4 -Measurement total T4 includes both the bound and free hormone. Changes in the levels of thyroid hormone-binding serum proteins cause corresponding changes in the total T4 concentration, even if the level of the physiological free T4 are not changed. Thus, a patient may be clinically and physiologically healthy, but exhibit pathological values ??of the total T4 concentration. Free T4 in serum can (Editor’s note: and should) be measured directly, with which one can avoid the pitfalls in the interpretation of total T4 concentration. Free-T4 index is a calculated value representing the total T4 concentration to the changes that are caused by different values ??for thyroid hormone-binding serum proteins corrected, and thus an estimate of the free T4 allowed if the total T4 is determined , The thyroid hormone binding ratio or in vitro T3 uptake is used to estimate the protein binding. The index for free T4 is easy to obtain and correlates well with the direct measurement of free T4.T 3 measurement serum total T3 and free T3 can also be measured. Since the binding of T3 is very strong to TBG (although it is 10 times weaker than the binding of T4 to TBG), the serum total T3 -mirror both by changes in serum TBG levels as well as through drugs that inhibit the affect binding to TBG changed. The mirrors for freiesT3 in serum using the same direct and indirect methods (Free-T3-Index) as described for T4, bestimmt.Thyroxin Also TBG binding globulin may be measured. It is increased in pregnancy, with oral estrogen therapy, or use of oral contraceptives and in the acute phase of infectious hepatitis. TBG can be increased as a result of X-linked variants or decreased (familial TBG excess or deficiency) be. It is most commonly reduced by diseases that reduce the hepatic protein synthesis, through the use of anabolic steroids and by the excessive use of corticosteroids. High doses of certain medications such. As phenytoin or acetylsalicylic acid and its derivatives displace T4 from its binding site of TBG, which are autoantibodies to thyroid peroxidase to an apparent decrease in the total serum levels of T4 führt.Autoantikörper against thyroid peroxidase in almost all patients with Hashimoto’s thyroiditis have some of them ( antibodies to thyroglobulin), and in most patients with Graves’ disease detectable. These antibodies are considered markers of autoimmune disease, but does not cause most likely. Antibody to the TSH receptor on the thyroid follicle are responsible for hyperthyroidism in Graves’ disease. Antibodies to T4 and T3 can be found with an autoimmune disease of the thyroid very rare in patients and can use the measurements of T4 and T3beeinflussen.Thyroglobulin The thyroid gland is the only source of thyroglobulin, which easily detectable and usually in patients with toxic serum of healthy people and nontoxic goiter is increased. In general, the determination of Serumthyreoglobulin in monitoring of patients with differentiated thyroid carcinoma after total thyroidectomy is used (with or without 131I ablation). Normal or elevated Serumthyreoglobulinwerte show Residuales normal or tumorous functionally active thyroid tissue in patients receiving the TSH-suppressive doses of L-Thyroxine or after withdrawal of L-thyroxine. The presence of antibodies to thyroglobulin affects the Thyreoglobulinbestimmung.Untersuchung on thyroid dysfunction For all the men and ? 65 for all women ? 35 and for pregnant women, screening is recommended every 5 years, in which the serum TSH levels are measured. Are risk factors for thyroid disease before, serum TSH should be checked more often. Screening for hypothyroidism is as cost-effective as screening for hypertension, hypercholesterolemia, and breast cancer. This single test is very sensitive and specific for the diagnosis or exclusion of two widespread and serious diseases (hypothyroidism and hyperthyroidism), both of which can be treated effectively. Due to the high incidence of hypothyroidism in the elderly, is a screening on an annual basis for persons> 70 years, appropriate. Imaging with radionuclide uptake of radioactive iodine can also be measured. A small amount of radioactive iodine is administered orally or i.v. given. Using a scanner, the amount is determined to radioiodine which is absorbed by the thyroid gland. The optimum radioactive iodine isotope would 123I, which exposes the patient only a minimal radiation (significantly less than 131I). The thyroid uptake of 123I is highly dependent on the previous iodine intake and low in patients who have taken excessive iodine. This test is valuable in distinguishing the various causes of hyperthyroidism (high recording in Graves’ disease, low consumption at a thyroiditis – Hyperthyroidism: Diagnosis, Editor’s note: in this indication the increased / decreased vascularization of the thyroid gland in the color Doppler sonography is at least equivalent) and indispensable for the diagnosis of functional autonomy. It can be helpful in dose finding a 131I therapy to treat hyperthyroidism. After application of the radioisotope scintigraphy is performed (with radioiodine or technetium-99m pertechnetate) to obtain a graphical representation of intrathyroid nuclide. Foci of increased (hot, hyper-functional) or reduced (cold, hypo-functional) recording serving the differential diagnosis thyroidal node (thyroid carcinomas come in <1% hot node before compared to 10-20% in cold nodes).