Source: USGS Landsat Mission

In the upcoming weeks, the Landsat Project will incorporate new radiometric enhancements for all Multi-Spectral Scanner (MSS) and Landsat 4 Thematic Mapper (TM) thermal data. A notice will be posted when the new calibrations are released in early April. At that time, all processed MSS and Landsat 4 TM data will be purged from the servers in order to ensure that all future MSS and L4 TM data have the new calibrations applied.

USGS Landsat Mission Website