The Belgian doctor said that FIFA already allows referees to order a drinks break in each half when temperatures top 32 degrees (89.6 Fahrenheit).
FIFPro secretary general Theo van Seggelen said the union gave FIFA a research document on the issue this week.
"We gave them some facts and figures, and let's see if they are going to take it seriously. I think they will," van Seggelen told the AP. "We have a good relationship with the medical committee so we try at least to explain the problem and exchange information."
D'Hooghe said changing the kickoff times was unlikely, after the match schedule was worked out with Brazilian football and government authorities more than two years ago.
"I respect FIFPro but I think we take care of the health of our players," he said. "We can't say you will always play between 17 and 20 degrees.
"There will be matches in warmer temperatures and colder temperatures. This is one of the difficulties in Brazil when the distances are so big. There are countries who are participating who are used to that."