Das basale TSH – Grundlagen und aktuelle Interpretation, basierend auf neuen epidemiologischen Daten

Determination of the basal thyrotropin (TSH) level is fundamental part of all thyroid examinations. As even subclinical thyroid diseases frequently cause symptoms or lead to a higher cardiovascular risk and thus require treatment, the understanding of this parameter is substantial. For accurately identifying thyroid disorders, several physiological mechanisms influencing the TSH level have to be taken into consideration. In addition to the well-known circadian and pulsatile rhythm of the TSH secretion or the changes in the hormone status and binding capacity during pregnancy, severe non-thyroidal illness, medication and nutrition can affect the TSH concentration. On the other hand, the reference range, which is currently intensely debated and subject of several new population based studies, is essential for the interpretation of the TSH measurement. In the US and other regions with sufficient iodine intake for a long time, the availability of more sensitive assays for detecting thyroid autoantibodies revealed an increased frequency of autoimmune thyroid disorders in subjects with basal TSH levels in the upper reference range. Further, this population proved to have a significantly increased risk of developing overt hypothyroidism. Therefore, the upper limit of the TSH reference range was reduced in these areas to exclude the cases with occult thyroid autoimmunity. In Germany, the iodine intake was improved only in the last decade, which independently led to an increase of the median of the TSH distribution curve in the formerly iodine deficient population and thus a “right-shift” of the reference range. Apart from physiological basics on the TSH and an overview of the laboratory procedures, this article describes the factors influencing the TSH concentration in vivo, which have to be taken into consideration when interpreting the laboratory findings, with a focus on the discussion about a possible adjustment of the lower and upper TSH reference limits considering the latest data. Further, a method for calculating the upper TSH reference limit in children depending on their age is presented.