Shoaib, who is in the eye of a storm over his refusal to take the field on the third day of the series-deciding Test against India at Rawalpindi last week, is among five cricketers asked to appear before the four-member panel constituted by the Pakistan Cricket Board (PCB).
The player's brother Shahid Ahktar said he had written to PCB informing them about Shoaib's inability to appear before the panel on Sunday.
"I have informed the Board in writing that Shoaib would not be able to attend Sunday's meeting and they should set another date in mutual understanding with him," Shahid was quoted as saying in local media reports in Islamabad on Thursday.
The other ciricketers who have been summoned to appear before the panel include Moin Khan, Abdul Razzaq, Shabbir Ahmed and Umar Gul.
The letter by Shahid said Shoaib had gone out of the country to honour commitments made by him well before the Board constituted the medical inquiry.
The fast bowler is expected to return to Pakistan early next week.
Shahid also expressed his anger at the negative publicity being accorded to the fast bowler and rubbished reports accusing the bowler of partying and drinking in Mumbai.
"No Indian media source has reported any such thing about Shoaib. We don't understand why efforts are on to malign him," he said.