His sister Najla has told the BBC his parents will never forgive him.
On social media, Dr Abuanza has said he wished that a Jordanian pilot burnt alive by IS had taken longer to die.
On his Facebook page he is pictured wearing doctors' scrubs and carrying a gun in a holster. He smiles as he raises his finger in the air - a symbolic gesture to represent the oneness of God that is commonly seen in the iconography of Islamic extremism.
Another image shows him in combat fatigues, cradling an automatic rifle and reading the Koran.
Dr Abuanza is a Palestinian doctor with British citizenship who spent seven years working for the NHS. He is the first practising NHS doctor known to have joined the Islamic State group.
Security minister John Hayes said IS "target vulnerable people".
He added: "They also target children, but they target professionals too. They are trying to corrupt British people of all types, encouraging them to murder and maim their neighbours and to go and fight in Syria."
'Dreadful enterprise'In a January 2015 Facebook post, Dr Abuanza celebrated the terrorist attack on the Parisian headquarters of the satirical magazine Charlie Hebdo in which 12 members of staff were gunned down, writing: "Praise be to God for this terrorist act. God kill off their enemies, military and civilian, men and women, adult and children.."
The following month he wrote about the killing by IS militants of a captured Jordanian pilot who was burnt alive, complaining that: "I would've liked for them to burn him extremely slowly and I could treat him so we could torch him once more."
The BBC has not been able to reach Dr Abuanza, who has not posted anything since October. He had crossed into Syria on 26 July 2014, soon after the creation of the Islamic State's self-proclaimed caliphate. Like thousands of other recruits to IS he completed a registration document. In this, he told his handlers he was a doctor specialising in endocrinology - the treatment of hormonal imbalances.
In Sheffield he had been combining shifts as a registrar with running an online clothing company selling kaftan dresses but had dissolved it three months before arriving in Syria.
His wife did not want to be interviewed, telling the BBC that no-one, including her, had had any idea of his plans.
Dr Abuanza had qualified as a doctor in Baghdad the year before the US-led invasion of Iraq in 2003. From May 2007 to July 2009 he worked at Glan Clwyd Hospital in Rhyl, North Wales, before moving around the UK with work.
He has been confirmed as working at Scarborough Hospital between October 2012 and August 2013. During this time he posted an online video of himself praying in the doctors' on-call room in the hospital.
Scarborough and Whitby MP Robert Goodwill expressed his shock that "somebody who came to Scarborough to work in the health service, to save lives and make people better is now engaged in this dreadful enterprise".
'Path to terror'His sister, Najla Abuanza, told the BBC: "He used to be quite the dashing young man, very modern. I've no idea how he became like this or who showed him the path to terror."
Her parents had become unwell because of the strain, she said. "They will never forgive him. My dad's wish was to see him before he dies. He has spent all his money on him and his education and this is what he does."
She even took to social media to berate her brother for leaving his wife and children in the UK, telling him: "You left them in the same country which is after you now." Soon after arriving in Syria, Dr Abuanza began chronicling his work as an IS medic on social media. In one post, he wrote: "We get a lot of spinal injuries which cause the paralysis of mujahideens [fighters] because we don't have spinal surgeons."
He also implored other Western medics to join him and when a group of British medical students abandoned their studies in Sudan and arrived in Syria in 2015, he wrote: "A couple of days ago our only emergency doctor had fled. I was extremely shocked and saddened. Suddenly, 11 Sudanese doctors entered Islamic State territory. You've no idea how happy I was."
It has not been possible to verify the authenticity of all of the IS recruitment papers seen by the BBC but many of the ones for British fighters have proved to be genuine.
Dr Abuanza was not the only professional among the British recruits; a third of them went to university and three say they were teachers.
Dr Erin Saltman, senior researcher with the Institute for Strategic Dialogue, said: "We shouldn't be surprised by professionals joining, since Islamic State is not just recruiting combatants. Less like al-Qaeda, this is more akin to Soviet or Nazi youth propaganda that says everyone has a part to play in building this Islamic, purist, utopian society."
The current whereabouts of Issam Abuanza are unknown, though in October 2015 he was living in Deir Ezzour province in eastern Syria. His startling journey from working the rounds in British hospital wards to the battlefields of Syria is yet another reminder of the dangerous appeal of Islamic State.