According to the media, the KNSB said that signals of increased use of hormones came from the medical staff during the inspection of athletes made during the previous season.
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Hence, the federation sent a circular letter to athletes, warning about the risk to health that occurs when using those drugs without medical indications. The KNSB also reportedly encouraged skaters to file complaints about doctors who help get access to hormones.
Athletes use thyroid hormones to quickly burn fat and reduce body weight. These drugs are not included in the Prohibited List of the World Anti-Doping Agency (WADA) but their uncontrolled usage may lead to health problems.