On Wednesday, after arguments erupted among lawmakers over the fate of the journalist Muntadhar al-Zeidi, the speaker said: "I have no honor leading this parliament and I announce my resignation."
Al-Zeidi's family went to the Central Criminal Court expecting to attend a court hearing for him, said his brother, Dhargham. He added they were told the investigative judge went to see him in jail and that they should return in eight days.
"That means my brother was severely beaten and they fear that his appearance could trigger anger at the court," he added.
Iraqi officials and another brother have denied that the journalist suffered severe injuries after he was wrestled to the floor after throwing the shoes during a press conference by Bush on Sunday.
The reporter shouted in Arabic, "This is your farewell kiss, you dog!" In Iraqi culture, throwing shoes at someone is a sign of deep contempt, and his actions have drawn huge demonstrations of support among many in Iraq and throughout the Arab world.
In parliament, lawmakers had gathered to review a resolution calling for all non-U.S. troops to withdraw from Iraq by the end of June but those loyal to radical Shiite cleric Muqtada al-Sadr interrupted the session.
They said parliament should focus on al-Zeidi's case rather than the proposed legislation. The argument escalated with lawmakers screaming at each other, and finally leading al-Mashhadani to announce his resignation, said Wisam al-Zubaidi, an adviser to Khalid al-Attiyah, parliament's deputy speaker.
An official in the speaker's office said he was unsure about the seriousness of al-Mashhadani's announcement, but says it may have been made because he was nervous. The official spoke on condition of anonymity because he was not authorized to talk to the media.
Al-Zeidi reportedly spoke glowingly of al-Sadr, whose followers organized protests Monday to demand his release.
Al-Mashhadani, a member of the minority Sunni faction, has previously threatened to quit and has been suspended for embarrassing Prime Minister Nouri al-Maliki with erratic behavior.
Al-Zeidi was held for allegedly insulting a foreign leader, a charge that carries a maximum of two years in prison.
Under the Iraqi legal system, the case is given to a judge who investigates the allegation, weighs the evidence and recommends whether to order a trial.
The process can take months, and it is normal for initial hearings to be conducted informally rather than in a formalized setting common in U.S. and British courts.
Thousands have taken to the streets in the days since al-Zeidi's arrest, heralding his actions and calling for his release.
About 1,500 demonstrators took to the streets Wednesday in the Baghdad Sunni neighborhood of Azamiyah to demand his release. Al-Zeidi was kidnapped in the same neighborhood last year and was freed unharmed a few days later.
"This is a natural reaction to the American acts of tyranny and occupation in Iraq," said demonstrator Khalil al-Obeidi a resident of Azamiyah said.
Shiite lawmaker Bahaa al-Araji said he expected al-Zeidi, who's in his late 20s, to be released on bail in the next few days while the investigative judge considers the case.
Associated Press Writer Sinan Salaheddin contributed to this report.