thermostat vs hold in

thermostat is a noun but hold in is not a noun.

Word	Noun	Adjective	Verb	Adverb
thermostat	Yes	No	Yes	No
hold in	No	No	Yes	No

As verbs, hold in is a hypernym of thermostat; that is, hold in is a word with a broader meaning than thermostat:

thermostat: control the temperature with a thermostat
hold in: lessen the intensity of; temper; hold in restraint; hold or keep within limits

Other hypernyms of thermostat include check, contain, control, curb, hold, moderate.

thermostat (noun)	hold in (noun)
a regulator for automatically regulating temperature by starting or stopping the supply of heat

thermostat (verb)	hold in (verb)
control the temperature with a thermostat	hold back; keep from being perceived by others
	lessen the intensity of; temper; hold in restraint; hold or keep within limits
	close in