Small medical thermometer
Quantity limits: One small clinical/medical mercury thermometer.
The thermometer must be in a protective case.
Regulation that allows it: 49 CFR 175.10(a)(7)
Tip: Thermometers with a red line instead of silver are not mercury and not restricted. Digital thermometers are also not restricted.
Though allowed in checked and carry-on baggage by U.S. regulations, medical mercury thermometers are not allowed in carry-on baggage by international (ICAO/IATA) regulations.