stiffen vs throttle

stiffen is not a noun while throttle is a noun.

Word	Noun	Adjective	Verb	Adverb
stiffen	No	No	Yes	No
throttle	Yes	No	Yes	No

As verbs, throttle is a hypernym of stiffen; that is, throttle is a word with a broader meaning than stiffen:

Other hypernyms of stiffen include bound, confine, limit, restrict, trammel.

stiffen (noun)	throttle (noun)
	a valve that regulates the supply of fuel to the engine
	a pedal that controls the throttle valve

stiffen (verb)	throttle (verb)
become stiff or stiffer	reduce the air supply
make stiff or stiffer	place limits on (extent or amount or access)
severely restrict in scope or extent	kill by squeezing the throat of so as to cut off the air