The Dutch skating association KNSB has written to the country’s top speed skaters warning them not to take thyroid hormones in a bid to improve their performance, the Volkskrant said on Thursday.

‘The KNSB believes in a clean, fair and doping-free sport,’ the letter, signed by technical director Remy de Wit, said.

‘We have had recent indications that athletes within the skating circuit are being prescribed drugs with are not on the WADA banned list but which have risks when taken without medical need. We want to warn you specifically about this.’

The KNSB did not name names or say how many skaters are involved, but three skaters and one coach told the Volkskrant they are aware of the rumours.

Thyroid hormones are used to stimulate slugging thyroids into working better, and can help lose weight and boost energy levels.

Wada

The Dutch doping authority has also written to world anti-doping agency Wada urging it to ban the drug, NOS said.

The letter, which was also signed by the Dutch sports ministry, states: ‘We have received indications from medical professionals that thyroid hormones are being abused by top Dutch athletes.’

Cyclists and triathletes are also said to be involved, NOS said.