Thyroid hormones

The thyroid is an important hormone gland and, in addition to its function as an iodine store, is responsible for the formation of the thyroid hormones triiodothyronine and tetraiodothyronine.

In the thyroid follicles, triiodtyronine (T3) and thyroxine (T4) are formed by iodination of the amino acid thyronine. The required iodine must be ingested via food. T3 and T4 are released into the peripheral blood at a ratio of 1:10, where they are bound to the serum proteins to a degree of over 99%. However, both hormones can only be absorbed into the target cells in an unbound, free form as fT3 or fT4, whereby fT3 is the actual metabolism-active hormone. While T4 is produced only in the thyroid gland, about 80% of T3 is produced in the periphery by conversion of T4. Since this deiodination mainly occurs in the liver, it is understandable why an impaired liver function can lead to changes in the thyroid metabolism. Reverse T3 (rT3) is a biologically ineffective metabolite, which is mainly produced when the need for thyroid hormones is low or in the course of a T4 to T3 conversion disorder. Using the analysis of rT3, the additional determination of the total T3 (TT3) amount as well as the determination of the TT3/rT3 ratio offers improved sensitivity in the context of extended thyroid diagnostics. It is therefore possible to obtain clear indications of a conversion disorder, which could be of significance for non-thyroid diseases or unsuccessful hormone therapy with T4 monopreparations. Another important hormone is TSH (thyroid-stimulating hormone, thyrotropin) produced in the pituitary gland. It stimulates growth, iodine uptake and hormone production in the thyroid.

In addition, the determination of antibodies involved in thyroid activity can provide diagnostic support. TSH receptor autoantibodies (TRAb) can bind to the TSH receptor and cause various clinical symptoms. Thyroperoxidase (TPO-Ab) antibodies are antibodies directed against thyroperoxidase and indicate various autoimmune thyroid diseases.