Both UK Anti-Doping and the US Anti-Doping Agency have lobbied for the inclusion of thyroid medication in the list, which is reviewed annually, claiming it can increase metabolism and act as a stimulant, as well as enhancing weight loss.

UK Anti-Doping said it was "disappointed" by WADA’s decision.

Earlier this year, Britain’s European 10,000m champion Jo Pavey claimed some athletes were using “unethical” thyroid medication to enhance performance.

"Some might be legitimate but I find the situation worrying," she said.

In reponse to the WADA decision, Pavey tweeted today: "The use of thyroid medication without a medical need clearly goes against the spirit of sport.”

WADA spokesman Ben Nichols commented: "For a substance or method to be prohibited, it must meet two of three criteria: enhance performance; pose a threat to athlete health; violate the spirit of sport."

He added that Wada had consulted scientific and medical experts who "were unanimous in their view" that thyroid medication did not meet the criteria needed to ban it.

Explaining why the decision had left her organisation "disappointed", UKAD chief executive Nicole Sapstead said: "UKAD formed the view, along with a number of other national anti-doping organisations, that thyroid medication, if used without a genuine medical need, is harmful to health.

"It can be used in a manner which is contrary to the spirit of sport and, in some circumstances, can be considered performance enhancing.

"This is the third year that UKAD have submitted a proposal for thyroid medication to be added to the prohibited list.

"UKAD will continue to work with global partners and consider our next steps in due course."

Dr. Olivier Rabin, science director for WADA, told the Wall Street Journal that the Expert Committee reviewing recommendations to the prohibited list had been analysing research as recently as late August before arriving at their decision not to include thyroid hormone.

“All the experts in the field came to the conclusion that no, there is no way to believe that thyroid hormone could be performance enhancing,” he said.

After reviewing their evidence, Dr. Rabin said that members of WADA’s Expert Committee were near consensus that thyroid hormone does not significantly pose harm to health, and doesn't provide any performance enhancement effect.

Furthermore, he said, unlike substances with clear performance-boosting benefits like EPO, the range of what is perceived to be normal thyroid function varies from person to person and is difficult to standardise.

Dr. Rabin added that the prevalence of thyroid hormone use in sport belongs in what he calls the “medicaliation of performance,” whereby athletes pursue advanced medical therapies in an effort to gain an edge, even if those therapies don't contribute to any clear performance enhancement.

“We monitor medicalisation of performance because it can be a step into doping, but it doesn’t mean its doping,” he said.

Changes to the 2016 banned substance list, which will take effect on January 1 and will be in effect for the 2016 Summer Olympics in Rio, include the removal of international motocross from the list of sports for which alcohol use is prohibited.

Other changes include addition of meldonium and insulin-mimetics as banned substances.

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