The ornamental design for a thermometersheath, as shown and described.
1. Thermometer sheath
Fig. 1.1 is a bottom right perspective view of the thermometer sheath; fig. 1.2 is a front side view of the thermometer sheath; fig. 1.3 is a rear side view of the thermometer sheath; fig. 1.4 is a left side view of the thermometer sheath; fig. 1.5 is a right side view of the thermometer sheath; fig. 1.6 is a top side view of the thermometer sheath; fig. 1.7 is a bottom side view of the thermometer sheath.