B787 First Officer reported uncommanded go-around during final approach.

Narrative

On approach into ZZZ; ILS PRM (Precision Runway Monitor) XXC; hand flown with auto throttles engaged. Vectored close behind another aircraft on the same approach. The airplane was configured flaps 20 with the gear up. Used speed brake to manage the ATC assigned airspeed of 170 kts.I retracted the speed brake to the stowed detent and moved my hand to the throttles. As I was moving my hand and neither touching the speed brake lever or the throttles; I visually noticed the flight director on the HUD move to a climb attitude and the FMAs change to THR/LNAV/TOGA. I moved my hand down to stop the throttles but they had advanced past where I had expected them to be; at which time I moved my head to visually see them advanced forward. I began to move the throttles back to maintain the approach and fly it raw data; but decided that I would become unstable. I told the Captain that we were going around. The additional thrust had caused the airplane to start to climb; so instead of unloading the airplane I stopped the nose up movement and made the go around calls. The Captain informed the Tower of the go around. We were instructed to maintain 6000 ft. We had already climbed through that altitude and began a descent back to it. I reengaged the autopilot and the Captain ran the checklists. ATC vectored us around for another ILS PRM XXC. After a brief discussion; I hand flew the approach with the autothrottles disengaged to a normal landing. The Captain submitted a discrepancy 'AUTOFLT SYS DIRECTED UNCOMMANDED GA DURING APPROACH'. An avionic Technician met the airplane to discuss with us what had happened. We briefed him and left to fly our next leg.