A thin film transistor substrate allowing an increase in proportionate size of the display area includes gate driving units, signal test pads, a first function line, and a first-stage gate output signal line in a surrounding non-display area, the display area having pixels. The gate driving units include a first-stage gate driving unit with an output terminal electrically connected to one row of pixels. The first function line has a first function but can also multiplex as a connecting line between the first-stage gate output signal line and one test pad. The first-stage gate output signal line is electrically insulated from the first function line during the first function of the first function line but is electrically connected to the first function line when multiplexed.