Thyroid medication is probably not the reason for the records breaking, IMHO.
I was more interested in the fact that apparently riders spoke to the press about it.
A tailor made cocktail is probably what is being used - cortisone is the other substance mentioned in the article.
(the article in dutch is
here the important bits,
"Around the opening weekend in Flanders, a number of cyclists contacted
WielerFlits to indicate that they are aware that Thyrax is used in the peloton today. It does not directly refer to teams or riders, but it is explicitly indicated that several riders at the highest level use the drug during training camps in the run-up to important races.
...
The cycling peloton also points to the use of the 'cocktail' Thyrax and cortisone in the preparation for races, where both products would have a strengthening effect on each other.)