A friend of mine, has a mare that recently gave birth to her foal. The box stall in which the pair is located, has very low light, and seeing that the foal is nervous, and the mother quite over-bearing, we decided not to make use of a flash, and instead, crank up the ISO. In the stall, we used 12,800 ISO, and in the ring (where they were out running and burning off excess energy), we cranked it up even higher (20,000 ISO), as to gain a little more shutter speed.
Grant you, these aren't extreme conditions, but rare are the cases where I'm subjected to using an ISO setting higher than 3,200. To be honest, I'm quite surprised with the results. With very little noise reduction and "balancing", the images are more than "usable". (see both the "Before & After" images below)
Follow this LINK, and see a wider variety of results for yourself.
Here's the Original - 20,000 ISO / F2.8 / 1/250