burthen vs burden

burthen and burden both are nouns.

Word	Noun	Adjective	Verb	Adverb
burthen	Yes	No	Yes	No
burden	Yes	No	Yes	No

As nouns, burden is a hypernym of burthen; that is, burden is a word with a broader meaning than burthen:

Other hypernyms of burthen include load, loading.

As verbs, burthen and burden are synonyms defined as:

Other synonyms of burthen include weight, weight down.

burthen (noun)	burden (noun)
a variant of `burden'	weight to be borne or conveyed
	the central idea that is expanded in a document or discourse
	an onerous or difficult concern
	the central meaning or theme of a speech or literary work

burthen (verb)	burden (verb)
weight down with a load	impose a task upon, assign a responsibility to
	weight down with a load