It took a second-half header from James Perch to pull Forest level after James McFadden had put the visitors ahead.
But after the midfielder's leveller it was all Forest, with Nathan Tyson, Joe Garner and Chris Cohen all coming close to snatching victory for the home side as Birmingham found themselves under a barrage of late pressure.
The energetic Paul Anderson had twice tested Birmingham keeper Maik Taylor with stinging shots as Forest made a lively start.
But it was Birmingham who took the lead following a questionable free-kick as Kelvin Wilson was harshly punished for a high boot as Sebastian Larsson ducked down to head the ball.
The visitors lifted the free-kick into the box where McFadden ghosted in free at the far post and, having had an initial shot saved by Lee Camp, then fired the rebound through the keeper's legs from a tight angle.
It took Forest 20 minutes to force a meaningful response, with Tyson inches away from winning a penalty as Radhi Jaidi hauled him down right on the edge of the box and earned a booking.
Wes Morgan then strode forwards from the centre of defence and beat two Birmingham players on the edge of the opposition box before seeing a shot charged down, with Garner then forcing a smart save from Taylor after Anderson had whipped the loose ball back into the danger zone.
Forest's equaliser came six minutes into the second half as Cohen delivered a free-kick to the far side of the box where it picked out Luke Chambers.
The defender fired an instant shot that was flying wide until Perch threw himself bravely to head the ball inside the far post.
A Lee Carsley free-kick gave Forest a scare as a deflection forced Camp into a save, but Taylor had to react well at the other end to thwart Tyson as he hit a shot on the turn, while Thornhill drove over after more good work from Tyson.
Birmingham were still a threat as well, however, and Marcus Bent should have restored their lead but headed a great chance over from eight yards.
Tyson too was guilty of missing a gilt-edged opportunity as he latched onto a Cohen flick-on but side-footed the ball wide from close range.
But Birmingham held on for a point, as the Reds were made to rue their missed chances.