A hand-written note at the bottom of the Prime Minister's letter to Mrs Fatima Khan says "You are in our thoughts and prayers".
The letter, dated December 20, calls Dr Khan's death a "sickening and appalling tragedy" and adds that "it is right that the Syrian regime should answer for it".
David Cameron has written to the mother of a British doctor who died in custody in Syria, describing his death as "a sickening tragedy".
The mother of British doctor Abbas Khan, who died after 13 months in a Syrian prison, has accused the regime of "murdering" her son.
Relatives of British surgeon Dr Abbas Khan have said they do not understand what led to his death just days before he was due to be released