What's the amount of Mercury and Thallium in a typical low-temperature thermalmeter?
How poisonous is Thallium when bounded with Mercury?
The lowest eutectic in the Hg-Tl system occurs at 8.6 at.% Tl at 213K (see W. Gierlotka et al., CALPHAD 30425-430 (2006)). This is about 21K lower than the freezing point of pure Hg.
For toxicity, refer to an SDS (MSDS) or ask you local ES&H or industrial hygiene professional.