Chris Horner’s aim of defending his Vuelta a España title has derailed even before the race has started, with the American rider blocked from taking part due to a drop in his cortisol levels.

Horner’s Lampre Merida team is a member of the Mouvement Pour Un Cyclisme Crédible (MPCC) anti-doping organisation. It includes a requirement that teams rest all riders with cortisol levels below a certain threshold, a measure which it introduced in the past on health grounds.

Cortisol is a naturally-occurring hormone in the body and has a variety of functions. The MPCC states that suppressed cortisol levels can have health implications for riders, particularly if they crash, and teams which have signed up to the body agree to rest any riders who are under the defined MPCC threshold.

Cortisol levels can be suppressed for a number of reasons, including the use of cortisone or similar substances.

In Horner’s case, the rider and his team are blaming the lung problems he has had this year plus the medication he has had to take.

Team doctor Carlo Guardascione explained the situation in a Lampre-Merida statement. “After the finish of Tour de France and after the Tour of Utah where the athlete was still suffering from bronchitis, Chris Horner underwent two examinations by two specialists for his bronchitis as he had been suffering since the beginning of the Tour de France as well as during the Tour of Utah.

“Both specialists agreed that a treatment of cortisone by oral means was the only way to resolve this problem. All the necessary steps were taken to request a TUE (therapeutical use exemption). This authorization was given by UCI commission for the athlete to proceed with this therapy on the 15.08.2014.

“Physiologically this treatment can cause a lowering of the cortisol, together with other factors such as jet lag after his travel from United States where he had a time difference of 9 hours.”

He said that pre-Vuelta blood tests on riders by the UCI showed that Horner’s cortisol level was below the level deemed acceptable by the MPCC. “Thus the decision from the team to not allow the athlete to partake in this Vuelta, even with having all the necessary UCI authorisation in order.”

The team said that while the race was a very important target for Horner, that it would respect the rules and regulations of the MPCC and not start the rider.

The subject is a complex one as while many WorldTour teams are part of the MPCC, others are not. Chris Froome’s Sky team is not a member, and was able to get permission for the rider to use an oral corticosteroid during this year’s Tour de Romandie as he had a lung infection.

He went on to win that race, but he, the team and the UCI faced claims by the Journal du Dimanche newspaper that the correct procedures were not followed. It said that the UCI’s scientific advisor Dr. Mario Zorzoli had granted the request without submitting Froome’s medical dossier to a TUE committee.

WADA told CyclingTips that it was satisfied that the UCI had followed the rules of the International Standard for Therapeutic Use Exemptions (ISTUE) and that it would not be pursuing the matter further.

However there were unconfirmed reports that the agency was privately unhappy with the UCI for not having a TUE committee in place. The governing body subsequently said that it would be using such a panel in future cases.

Had Sky been part of the MPCC, Froome would have been unable to use the substance in competition.

The news about Horner brings the question about MPCC membership back into the spotlight, with the member teams voluntarily accepting stricter rules than those squad which have elected not to join.

The Belkin Pro Cycling team is another member and last year agreed not to start Theo Bos in the Vuelta after his cortisol levels were too low. The Dutch sprinter blamed asthma medication for the issue.

It is unclear as to what Horner will do next. CyclingTips contacted his agent Baden Cooke, but the Australian said that he had not yet spoken to the 2013 Vuelta champion and so it was unclear if he would have an alternative race programme in the coming weeks.

If he does, it will be necessary to show that his cortisol level has returned to the normal range.

Horner said that he was sad about the developments. “I was willing to try to defend the 2013 title, the Vuelta was my main target in the season, the team signed my with the aim of being competitive in the Spanish race, but I accept the decision linked to the MPCC’s rules.

“This bad bronchitis caused me a lot of problems. I’ve been suffering for it for weeks and this treatment could have allowed me to solve the problem. The UCI gave authorization for the treatment, I could race according UCI rules, but my team is member of MPCC. I understand it and we all must accept this situation without regrets.”