Endocrine Abstracts

The Hypothalamic Pituitary Thyroid (HPT) axis is well-known as a classic endocrine negative feedback loop: increased levels of thyroxine (T4) in the blood are associated with a reduction of thyroid stimulating hormone (TSH) production and vice versa. However, the HPT axis is sensitive to alterations by a number of endocrine-disrupting chemicals (EDCs). Exposure to specific EDCs can deregulate the negative feedback loop and alter the TH/TSH levels. The intrinsic mechanisms for such deregulations are still unclear. Our aim is to investigate the different TH/TSH patterns associated with EDC exposures in different population groups. Therefore, we conducted an extensive systematic literature search of peer-reviewed epidemiological studies. Based on our eligibility criteria, we selected studies where the authors included TH/TSH measurements in pregnant women, workers or the general population, and the levels of the target chemicals were characterised in the corresponding human sample. We retrieved more than 25,000 studies, of which 532 were further assessed. Cross-sectional, case-control and cohort studies were the predominant study designs. A large range of chemicals were included with the predominance of heavy metals, per- and polyfluoroalkyl substances, bisphenols, phthalates, pesticides, polybrominated and polychlorinated compounds. Our results showed that the idealized view of the HPT axis is not supported by the evidence when EDCs are involved. Some EDCs induced reductions or increments of T4 and/or T3 levels but the expected TSH increase or decrease did not materialise. Similarly, changes in TSH levels did not induce alterations in T4 and/or T3. The mechanisms by which various EDCs disrupt the HPT axis and induce such patterns have not yet been fully identified. For this reason, the complexity of the EDC mode of action needs to be considered in order to identify chemicals that are potentially damaging to human health. Acknowledgement: The ATHENA project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 825161.