At the northern edge of Hillsboro, the highway has an interchange with Spur 579, which provides access to I-35, as well as US 77, just south of the point at which the freeway splits into I-35W and I-35E.

SH 81 was designated to replace the segment in Hill and Johnson counties, and represents the longest section south of Fort Worth with which US 81 did not have a concurrency with Interstate 35.

SH 81 was instead reassigned to a road from Llano to Brady (formerly Texas State Highway no.

On October 7, 1939, SH 81 was cancelled and returned to McCulloch County (as per the original minute order designating this road).

Media related to Texas State Highway 81 at Wikimedia Commons