CDU politician Andreas Schockenhoff died suddenly at the weekend of a heart attack aged 57, prompting speculation of possible Russian involvement due to his demand for escalating sanctions against the Russian government over the Ukraine conflict.
The state prosecutor in his home district of Ravensburg, Karl-Josef Diehl, requested the autopsy to head off "conspiracy theories" on the cause of death.
Diehl said there was "no evidence of third party involvement in the death of Dr Schockenhoff".
He added that "preliminary" results indicated that Schockenhoff had died of "cardiovascular failure while using the sauna in his home late Saturday, or possibly fell asleep there and died of massive heat exposure".
But he added that he was still waiting for the results of a toxicology report expected in the coming days.
Diehl had said the investigation had been carried out because "one could expect conspiracy theories, doubts and speculation to surface later" given Schockenhoff's "line of work".
Schockenhoff was deputy parliamentary leader of Chancellor Angela Merkel's conservative party, and coordinated German policy on Russia from 2006 until earlier this year.
He made repeated criticisms of the Kremlin for its tough stance on pro-democracy activists and the protest band Pussy Riot.
In October 2012, Russian officials refused to meet with him in Moscow, citing "defamatory" statements he made.
During this year's Ukraine conflict, he repeatedly argued for tough sanctions against Russia.
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